On the list of watches that I've owned (and also reviewed), Seiko is the brand name that shows up most often. Regular readers of the blog may have followed my climb up the Seiko food chain from entry-level SRP Seiko divers with the 4R series movement, to the SPB series with the 6R and its improvements over the less expensive models.
My next planned stop in the Seiko hierarchy was the watch that the community dubbed the "MM300," but I ended up jumping a few levels when I made an unplanned purchase of a Prospex LX Spring Drive last year. I had thought the similar size and case meant that I could mentally skip over experiencing the MM300, but I could not get that variant out of my head (plus that Sky LE GMT is not a diver).
I suddenly found myself setting up alerts for some of the more interesting variants of the MM. The most recent models (no longer branded as "MarineMaster") have been upgraded over previous versions with sapphire crystals and ceramic bezel inserts, so I specifically searched for one of those models. Also, I wanted something a little bit different, so I looked for one of the LE colorways that Seiko has produced. What I found is a "Black Series" SBDX033, which is one of a 600 watch Limited Edition released in 2020. Although mine carries the Japanense-market model number, I'll refer to it throughout the article by the U.S. model number SLA035.
UPDATE: If you'd like to read how the SLA035 compares to another big and black technical diver, check out the comparison to the Sinn U1 S here.
Although the current MM300 is still styled after Seiko's 1967 6215-7000, the modern iteration does not look to the uninitiated as a vintage remake. This all-black version with red accents looks especially contemporary.
The sheer size also helps. Typically we think of vintage watches, even divers, as being smaller than their contemporaries, but this watch is wide and thick at 44.3 x 15.4mm.
Wearing the SLA035
Similarly to the Prospex LX, the MM300 wears much more comfortably than its dimensions would indicate. The lugs curve downward against the wrist, and the back of the case is relatively flush. Despite the fact that it is only a half mm smaller in width and length than the LX, the SLA035 is actually quite a bit smaller visually. This can be attributed to the bezel having a much smaller diameter, as well as the crystal and dial being smaller. The bezel is under 42mm wide, and I would say that the MM300, when viewed from the top, comes off as being a 42mm watch. I also find that all-black watches also tend to shrink in appearance a bit.
The MM300 does not hide its visual thickness quite as well, however. The thin band on the side of the case helps, but it is 0.7mm thicker than the already thick LX, and it looks it due to the thick bezel.
It does fit under a sleeve...if you're wearing a sweater!
It also doesn't hide its weight. Compared to the titanium LX, the all-steel MM300 is noticeably heavier.
The Monocoque Case
One of the most unique aspects of the MM300 (also carried over from the original 6215-7000) is its monocoque case. As with Seiko's top Tuna models, there is no removable caseback. The movement/dial assembly is loaded from the front. In addition to the technical benefits, this creates a unique look for the case and also gives the watch more personality.
Despite this, the water-resistance of the MM300 is "only" 300 meters (hence the name)? While this is more than enough for...well...anyone, I feel that a watch this thick, and with such a sophisticated case design, could have a bit more water resistance. Seiko's Tuna models (and the similarly sized and priced Sinn U1) offer 1000m for example, and even the less expensive Synchron Military offered 300m. Sure, it doesn't really matter, but this is one of Seiko's flagship technical divers.
One other benefit of the one-piece case (for a couple of people anyway) is that the MM300 is supposedly impervious to helium entering the case. This allows it to be certified as a saturation diver without the need for a costly, complicated, and some would say ugly, helium escape valve.
In addition to that unique feature, the cases on all MarineMasters are exceptionally finished. Sharp intersections exist between the polished and brushed surfaces, and Seiko's famous zaratsu polishing is employed here. The black finish hides the zaratsu luster, but it still looks attractive, and I find the overall presentation very pleasing to look at. The case so black and polished that it looks like it's made from black ceramic.
As is common for a Seiko diver, the crown is offset at 4 to reduce wrist interference. Silver-cased models have a Prospex logo on the crown, but unfortunately, it is unsigned on all-black models like this and the SLA053.
The Bezel is the easiest to identify upgrade to the new generation MM300 watches. Not only is it now made of ceramic, but the full triangle at the zero position is filled with lume paint, as are the markers up to 20 minutes.
It tapers upward slightly and is quite easy to grip and turn. I was hoping for a more mechanical click feel, but it is fairly fluid like other Seiko divers. It feels more precise, and has much less play between the markers. This example also lines up correctly at 12.
The Sapphire crystal is one of my favorite aspects of this watch. The top surface is flat, but it uses a deep inner dome. It creates a distortion that is fun to look at, Also, unlike an outer domed crystal, it actually helps with reading the time at obtuse angles. Despite how deeply recessed the dial is, the crystal distorts it upwards when viewed from the side. It's still not as legible as a thinner watch with a flat crystal, but it makes it more interesting to look at.
If any actual divers are reading this, I'd love to know if this also helps reading the watch underwater at angles compared to a flat crystal.
I do wish that the crystal had the same fantastic AR coating on the outer surface that my Prospex LX has. Seiko also only applies it to the underside on its high-end dive watches though. It is still very effective, but the crystal doesn't disappear as it does on the LX.
Dial and Hands
One complaint from many die-hard Seiko fans was the loss of the MarineMaster nomenclature on the dial and its replacement with the Prospex X logo. While I generally prefer brands to avoid dial clutter as much as possible, this swap doesn't bother me. The MarineMaster font looked dated, and this logo takes up less transverse real estate on the dial. Additionally, I love the hint of red in the depth rating on this LE.
Although I usually prefer (no, demand) a black date disc on a black dial watch, this silver application does not bother me that much. It reads visually as another hour marker, and I like the clean white outline around it.
Also, I assume because this is a 2020 model, Seiko hasn't crammed in a lume marker to the right of the date as it has on the latest MM300 models.
I also find the hand designs pleasing. Seiko stuck with the vintage shape, rather than trying to make them too funky. They even have a thin polished chamfer on the sides of the brushed top surfaces (something that I complained was lacking on the Prospex LX). The red second hand also looks well-finished with its black counter-balance and "stoplight" lume.
Oh, and of course Seiko's Lumibrite is killer. I expected as much from a Seiko diver, but this one is above and beyond any others that I've owned. It's often visible during the daytime if you're in just a hint of a shadow. Unfortunately, it is visibly unevenly applied on the bezel triangle and hour markers, possibly as a result of such a thick application.
All of the current silver-cased versions of the MM300 come with Seikos famous ratcheting clasp steel bracelet, and some also come with a silicone strap. There is no black bracelet option, however, so all-black models only come with the wave-style silicone strap.
I'm sure you'll take it with a grain of salt that the owner of a watch strap company is criticizing another watch strap. That being admitted, even before I started StrapHabit, I was never a fan of Seiko's newer silicone dive straps. The material itself is very thick, yet soft and comfortable, but it picks up dust and fuzz. I also don't prefer the metal keepers. They don't secure the end of the strap from flopping as well as a rubber keeper would, and Unlike rubber keepers, they also pick up scratches.
Luckily, Seiko employed its typical drilled lugs on the MM300, so it is easy to swap the strap out for something that I prefer. A 20mm lug width means that you'll have plenty of strap options from StrapHabit or any other strap company. It also helps to make the watch appear a bit smaller than a 22mm width would have.
Removing the factory strap also reduces the visual heft of the watch, as it flares out to meet the edges of the lugs. Straight 20mm straps make it look a bit smaller.
Many Seiko fans tout that the MM300's 8L35 movement is a huge upgrade as it is an unfinished and unregulated version of the Grand Seiko 9S55. While it is a nicer movement than a 6R, the finishing and regulation are a lot of what makes the GS version the GS version.
Compared to a 6R35, it has a higher beat rate (8 beats/s vs 6), but also 20 hours less power reserve (50 vs. 70). The rated accuracy is also better than the 6R35's +25/-15, but not anything fantastic at +15/-10 seconds per day.
I would say that this movement can be compared to, but is slightly nicer than a higher grade ETA 2824/Sellita S200-1. It offers the same beat rate and similar accuracy. The power reserve is longer, but it gives up the instant date change. Both should be highly reliable, and the Seiko's movement gets the additional credibility of being made completely in-house.
Many collectors will agree that Seiko's (extensive) Prospex diver lineup has a number of holes in it. The roughly $500 watches starting with SRP offer a range of sizes and styles for a variety of tastes. Seiko fans can then move up to the SPB range and spend around $1,000 to see a nice increase in quality, still with a wide variety of choices.
The next step up the ladder to the roughly $3,000 SLA MM300 or the SLA "Willard" is a big one though. Those watches are also both a bit large and unconventional looking, thus not to everyone's taste.
Luckily for me, I don't shy away from big watches, which is why I made that jump to the "MM." If you're in the same boat, the SLA035 is worth a look. Swiss and German competing brands will sell you an endless supply of dive watches in this price range. With the SLA035, you get Seiko's 8L35, which is an upgrade from the ETA and Sellita movements that tend to occupy this price range (plus the Seiko movement is fully in-house). Additionally, Seiko's case finishing on its upper-end models is extremely nice, and better than a lot of competing watches.
Add this to its great wearability for the size, and interesting visual appeal and the SLA035 is worth a look. A lot of watch buyers would dismiss this watch because "who would pay that much for a Seiko?" but conversely, I find that appealing. I enjoy having a high-quality piece on my wrist that no one else will take notice of.
If you enjoyed this review, check out StrapHabit on Instagram for more photos of this watch, and other great content.
Also, stay tuned for a comparison to my other big, black dive watch, the Sinn U1 S! Can the MM300 dethrone it as the best diving watch available?
Do you think the SLA035 is worth nearly $3,000 MSRP? What diver would you choose over it? We'd love to hear in the comments.
Name: Seiko Prospex Black Series Limited Edition (a.k.a. MM300, a.k.a. MarineMaster 300)
Reference Number: SBDX033 (SLA035J1)
MSRP: $2,900 (Sold Out Limited Edition of 600 Pieces)
Dimensions: 44.3mm diameter, 50.5mm lug-to-lug, 15.4mm thick,
Lug Width: 20mm
Movement: Seiko 8L35
Power Reserve: 50 hours
Water Resistance: 300m
Weight: 118g (head only)
Crystal: Inner domed sapphire with inner AR coating
Bezel: Unidirectional rotating, 120 click with lumed ceramic insert