www.goiot.fr
EY installed oodles of these French deck fills in 1988. At that time, having a fill cap that used a winch handle was considered a rather cool idea, I replaced them all (waste, diesel, and water) with new SS ones, and IIRC they were a earlier version of the ones referenced in reply #3. Excellent quality SS product from White Water Marine.
I note that now the Goiot versions are offered in composite and not metal. (Our OEM parts may have been ss or aluminum, but after so many years am not certain.) Buying them from France might be a bit of an adventure, as well.
As pointed out above, thread types are a problem, and a friend of mine with a Catalina 350, has one odd looking plastic deck fill cap that the PO put on that sticks up a ways and was clearly sourced from somewhere not related to the factory part at all...
Since it's time and past-time to replace all the hoses anyway, just pull & replace that "historic" fitting as a part of the hose upgrade.
This does expand the project in the near term, but also takes it off the maintenance list for another 20 years. (sigh........)
Aside: your OEM fuel fill hose has likely started to permeate by now, and will contribute to a faint-but-constant smell inside areas adjacent; I learned this first hand.
________
I edited the thread title to make searching more productive, and hope the OP will understand.
