Poseidon Bund Quartz Dive Watch for German Kampfschwimmer

Current dive watch for Kampfschwimmers (German Navy SEALs).  (Seiko) quartz movement and sapphire crystal.  Notice the NSN on the dial.  I've heard these are supplied by Poseidon, a well known scuba dive equipment company.

500 meter water resistance?  Not sure how they test to that depth.