Seiko 6309 Series Dive Watches:
The 6309 divers replaced the 6105 as the standard models of 150m divers. The 6309s were marketed outside of Japan while a version using the 6306 movement was sold only in Japan (but for a relatively short 5 years). The cal. 6309 movement is a successor of the 6105. It has 17J and runs at 21,600 bph. It’s non-hacking and has a quickset day/date calendar mechanism.
Like the 6105, there are 2 series of 6309 divers. The earlier versions — 6309-7040 and 6309-7049 — are housed in big cushion cases and have round markers. The later versions — 6309-7290 and 6309-729A/729B — have slimmed-down cases and rectangular markers (which make them very similar to the current SKX007/173 models). While the earlier 6105 divers used the “turn and lock” crowns, the 6309 crowns are all screw-down. The 6309 divers seem to all have bi-directional non-locking bezels.
The 6309s were popular especially among American soldiers who bought them cheap from the PX, the 6105 divers were issued to one of the U.S. Navy Seal teams in the 1960s.