First I tried an older Rollei shade that I bought recently on eBay. The only marking was 34 (mm?). When it didn't fit, I returned it to the seller, telling him it's probably a Bay 3. (Now I wonder if I was wrong!) Then I ordered an inexpensive aftermarket shade (Fotodiox, I think), clearly advertised as Bay 2; it also is too large. I don't have another Rollei to compare it with, but my Planar lens seems smaller in diameter than Planars on 3.5F models. I'm assuming that's because mine is the earlier 5 element model. I know that Rollei made two sizes of the mirrored lens caps for the 3.5E. There's a smaller one for the version 1 (and I have that lens cap) and a larger one for the E2/E3. So you'd think it would have to be the same for the lens hoods. Anyway, I'm stumped.