I believe one version fits the II and III, and their -a and -b cousins, then another version fits the II and III -c and -f cameras.
One is ORAKO, and the other is OKARO. I don't remember which is which.
Both are hideously expensive.
My solution was to cut little discs of theatrical gel ( Roscolux R-21, 22, or 23), and tuck it under the lip of the RF window of my Leicas (did this on a 1932 II, and I think my 1950 III-f). A toothpick help manipulate the gel into place.
It achieved the same result ( increased contrast for the RF patch ) for free ( or the time it took me to achieve it ).
If you have friends who work in Theatre, they could probably fix you up with some scrap amber or yellow or rose gel...