I have some old (mid 1980's) Datamarine instruments (SL100 and SL200 models) located next to the companionway bulkhead on my boat. A 3.5" diameter hole was drilled through the bulkhead and it appears that the instrument is held in place by the front bezel flange and a circular ring on the back side of the bulkhead. I want to remove them but can't seem to get them out. The circular ring on the cabin side is held in place with 3 set screws. I've removed the set screws but the ring still cannot be removed from the back of the meter. Before I use additional force and risk breaking something I'll regret, does anyone know how these instruments are held in place? Thanks