Referring to the query about old type winch handles, I run into the same problem. I purchased a set of second hand Lewmar 40 winches in perfect condition. Year of production and identification number were etched on the underside as 72/300 and 72/924. The new type star-shaped winch handle heads which I already had on board, however, did not fit.
I contacted the factory and they informed me that the new standard of the winch handle head is 11/16" (measured on the 'square' part), while the older type measured 5/8". They are not available anymore.
I have been looking for a winch handle with a starshape with 5/8", but from another forum I learned that the older winch handles did not have a star shape
but were 15.8 mm square (post by Arend Schram on Dutch Albin Vega forum). This is identical to 5/8".
Your problem can be solved by machining down a new winch handle to 5/8" square. Because of the work involved it makes sense to start with a good quality floating winch handle and make sure the square locking plate remains intact. This would give you double insurance against loss.
PS If someone has an old type winch handle 5/8" square I would be interested. Would save me some work. Always good to have a spare on board.