I could be mistaken, but I believe the model 37 magazine is unique to that model, and not compatible with the later 5 series magazines. I shot a 37 many moons ago, and had several magazines and like the OP all the associated small bore competition stuff that went with it.
The rimfire 40x replaced the 37 in the Remington line up, and as it was a direct competitor to the much more popular Winchester 52 at the time, I doubt a whole lot were made. It was essentially purpose built for small bore bullseye competition which is all loaded single shot, so I doubt even shooters in the 1930’s/40’s bought many magazines, as there weren’t really required for the type of shooting the 37 was used for.
My best luck finding odd duck esoteric stuff, is going through those junk boxes most older gun shops have and that you see at gun shows. This is a fun way to maybe trip over that magazine or single shot adaptor in rust6 but restorable condition for “ gimme 10 bucks”