If it's for your Bessa R... it's going to have threads in the rear. Usually, if they're from the 50s and 60s, their filter size is 39mm. In fact, I'd dare say it's still 39mm. The Elmarit went from 39mm to 46mm filter size in its latest version.
I had an Elmar a while ago. Kinda funky lens. Mine was a 1936 vintage, uncoated and very good with B&W. I never put a filter on it, though... Good luck finding one! 🙂