Analog Digital Diver's 200 m with Data Transfer System

There is a slight discoloration on the sensor case
I have tried to wipe that off but then decided to leave it as is since it is on the back of the watch. Out of sight, out of mind :)
DON'T OPEN! SERVICE CENTER REPAIR ONLY. Well, I am not using the watch for SCUBA diving. Plus, if I replace the o-ring and lubricate it with a bit of watch silicone, I think I am on the safe side.
You can easily read out the movement information on the case back: C500 is the caliber number in this case. 5N0915 is the serial number. 5 stands for 1995 and N for November. The rest of the digits represent a running production number. This is the same notation as SEIKO uses. However, Citizen did not as consequently apply this system as SEIKO did. This is especially true for older Citizen watches.