SEIKO PROSPEX MARINEMASTER PROFESSIONAL BLACK SLA021J 300M DIVER AUTOMATIC MM300

A little more on it’s history, Seiko continued their development for a more robust diver’s watch, and in 1968 they announced the highly regarded ref. [phone removed by eBay]. This watch was a significant upgrade; with a massive mono block case that gives it a 300m water resistant, and also a Hi-beat Seiko caliber that allows it to be the first in the market back then. This is the exact same watch that the new black dial and bezel the new MM300 Black is based on.

The Seiko Marine Master Pro. Diver SLA021 is the successor of the highly acclaimed MM300 SBDX017, which was recently launched also with some much needed upgrades to keep it current. Overall, the new MM300 still retains the massive mono block case at 44.3mm with a height of 15.4mm. It retains the familiar refined angular lugs that is polished and satin brushed throughout. The finishing is done using Seiko’s traditional Zaratsu polishing method that is seen mainly on the Grand Seiko line, which creates a distortion-free mirror finish. The 300m water resistant case is treated with Seiko’s DiaShield coating on the case and bracelet. This provides scratch-protection and extra corrosion resistance.

The SLA021J features a high grade Seiko 26-jewel automatic, caliber 8L35, which is an undecorated and unadjusted version of the Grand Seiko movement, caliber 9S55. The 8L35 beats at 28,800 bph and has handwinding and hacking features. The movement has a 50 hours of power reserve with an expected accuracy tolerance of -10/+15 seconds per day.

The unidirectional bezel is made up of zirconia ceramic insert for the first time. Even better, the 12 o’clock triangle and the first 20 minutes markers (in 5 minutes increment) are all filled with Seiko’s proprietary Lumibrite luminescence that glows green. The bezel is raised with a coin-edge profile for easy operation. The Seiko’s signature screw-in crown at 4 o’clock.

Another new upgrade to the Marine Master Professional 300M Ref. SLA021J1 is the use of sapphire crystal with anti-reflective coating on the inside now. This is probably the most requested feature since the SBDX001 and finally answered by Seiko. It not only gives the watch the much needed scratch resistance but also improved clarity as compared to the hardlex used in the older models.

The MM300 comes with a satin brushed bracelet with polished details has a ratcheting micro-adjustment for adjustments on-the-go.

The Seiko Marine Master Professional 300M Ref. SLA021J will certainly be one of Seiko’s most iconic model following the success of the SBDX001 and SBDX017, that has the finishing and movement of the Grand Seiko inside out.