There are many vintage Seiko divers popular with collectors, and the diver here – a blacked-out 1999 Seiko GSX100 "Military" 7N21-0010 GSX (GSX100BBK-1) sport diver here, on it's original leather strap and with original Seiko 7N21 instructions manual, warranty paperwork, and inner/outer Seiko boxes – continues the Seiko reputation for building robust, stylish, and purpose-built divers for a wide-range of professional adventurers.
 
The Seiko GSX100BBK-1 here is not a standard Seiko model, but rather a timepiece from GSX Watch Japan. This Japanese watchmaker frequently collaborated with Seiko to produce unique, limited-production watches.  A respected Japanese boutique brand established in 1995 that frequently collaborates on limited-edition designs, GSX Watch Japan produced exclusive lines in partnership with popular anime and video game franchises (like Star Wars, Batman, Indiana Jones, and Resident Evil/Biohazard). 
 
The GSX 100 line here was created with a central theme of showcasing the beauty of high-quality leather, and features a simple, elegant enamel-style (horo-styled) finish with thick printed indices.  The straps were custom-made by the renowned Japanese leather manufacturer Wakuwakumoto, and utilize high-end, supple leather and features a moisture-resistant lining from Kuraray (Sofina Chal) for premium comfort.
 
These limited-run, collectible Japanese domestic market (JDM) watches like Seiko GSX 100 series here relies on Seiko Watch Corporation movements, usually utilize standard Seiko analog quartz, automatic, or kinetic calibers, to include the Seiko 4N21 movement here.  The company ceased releasing new products in 2018, but announced the end of its operations at the end of July 2020 and noted that, "in recent years it has become difficult to manufacture new products."
 
Seiko introduced its 7Nxx diver line in 1991, and it came in a range of dial colors, to include the black one here, and silver dial, among others.  The dial is stark and utilitarian, with military-esque hour indices on a pitch black glossy dial, and features a short arrow-pointed hour hand contrasted with a longer thinner minute hand, aiding in wearer ability to distinguish the two immediately.
 
It also benefits from the lack of a date function (hence no date window throwing off the sparse dial design), along the minutes track just below 4 o’clock, with negative impact upon the symmetry of its beautiful design.
 
Military forces the world over have long took note of Seiko’s dive watch lines, with their reliable movements – during its heyday and even today, Seiko divers are popular with various armed forces for this same reason, despite the widely held (false) assumption that military personnel only wear military watches issued to them.
 
With good reason, Seiko divers continue to have a tremendous cult following, despite Seiko's odd decision to discontinue its iconic legendary SKX divers in 2018 - Seiko dive designs are usually simple and robust, with strong evidence of Seiko divers that came before it – of which the 7N35 bears testament – to the instantly recognizable Seiko 7002 automatic, 7548 quartz, and even the famous 6309 diver.
 
Seiko debuted its SUS line in the early 1990s, after it perceived a disconnect between its offerings and younger consumers, in an attempt to wean the same customers off of digital quartz watches made by Seiko competitors.
 
Seiko set about conducting a series of surveys to gauge the interests of their target demographic, namely individuals between 19 and 22, by placing photographs of Seiko watches from 1913 through the early 1990s – and their overwhelming preference was the minimalist, utilitarian designs from Seiko’s wartime offerings.
 
And from, this the SUS (“Simple” and “Strong”) emerged.  The design utilized historical aviation, military, and railroad timepieces that emphasized legibility above all else (important considerations when designing military watches, as well).
 
The SUS line went on to become a popular watch in Japan, and Seiko produced them, with their multiple variants, for eight years, to include automatic, quartz, and kinetic movements.  Among these, the 4S15 “Military” variant – released in early 1996 – was by far the most popular. 
 
That said, the 4S15 was not actually designed to be a military watch (while the crown doesn’t screw down, it does hack).  But the 7N21 here does hack.  And it's nearly identical to the far more expensive 4S15 - see the last photo on this listing for a side-by-side comparison.
 
This Seiko diver comes on its original leather strap, and with nylon strap, rugged travel case, springbar tool, Seiko 7N21 instructions manual, warranty paperwork, and original inner/outer Seiko boxes.