Glad you found something at least. I had imagined is was on the door under the pressure plate and not the back of the pressure plate. As I said, a very dim memory. The person that might be able to help you is John Foster at www.biofos.com
, he has extensively researched Olympus Pen cameras, both viewfinder and SLR series. The film advance problem you mention is something that is not too uncommon in the FT/FV models. I don't know if it can be fixed in repair or indicates excessively worn parts. I have read that Maitani designed the original Pen F as a two stroke film advance/shutter charge to alleviate stress on the gears compared to a single long stroke. The Pen rotating sector shutter has a high spring pressure when wound. Even with my two stroke Pen F my thumb action is slow and deliberate when I wind on to the next frame.