My '88 boat had a fuel fill cap with a cast-in depression for a (short) winch handle stud. Worked OK, and obviated the need for a special opener tool.
EY installed these in the 80's. Ultimately we replaced all of the deck fill fittings with new ss models with "flip up" tabs that are easy to use.
Did you lose the OEM cap overboard?
ps: hope the OP does not mind, but I shortened the title to make searching a tad easier.
Speaking of "searching" here is just one prior thread about this topic.
Does anyone have a spare deck fill cap for the water or fuel fills from an 80's vintage Ericson. Lost my fuel cap overboard when the safety chain broke and all the replacements I can find are just a little too small. Most one's I can find have an outer thread diameter of 1 1/2", but the caps...
ericsonyachts.org