I figured you had the C version.
I also own a C version of the 35/2.5. It is a 39mm filter size. I am absolutely, 100% positive certain about this. Without any uncertainty, your lens has a 39mm filter thread. I just mounted a 39mm yellow filter on it to use on my M2 loaded with TMax 400. The Cameraquest site might be wrong or misleading (sorry Ko), I'll have to check. Regardless, if you're looking for a lens cap, it would be best to get it from Cameraquest. The OEM lenscap is a push-on cap that slips over the built-in hood. It looks just like the lens cap for many CV lenses, in fact they may be interchangeable (I haven't really attempted) but the lens cap for the 25 snapshot, the 15/4.5, and the 21/4 all look to be the same lens cap.....funny I haven't tried mixing them, I've been good about keeping the proper lens cap with the proper lens... Maybe I'll try one on the other, and so on. My guess is they will all fit each other. That's good news for you, because there is likely a "universal" lenscap for those older CV lenses that will hopefully still be available from Stephen (Cameraquest). Plus, you get to interact with Stephen, our head bartender and apparently all-around nice guy. If you can't get the lenscap from Stephen, and your lens is the typical C version 35/2.5, complete with the integral lens hood, I personally start trying various slip-on lens caps that I used to find in a big box at the local camera shop. Unfortunately, the local camera shop is a thing of the past. Very, very , unfortunate. Very unfortunate. I used to go in and ask to rummage through the "odds and ends" box which would contain all manner of filters, step-up rings, step-down rings, lens hoods, Kodak Series "X" filters and filter holder rings/adapters of all sorts, front lens caps, rear lens caps, body caps, T-adapters, reversing rings, Adaptall adapters, extension tubes, and the occasional and odd hotshoe adapter, PC flash cable, flash connector cover (PC or proprietary), Cokin and Lee square filters, Kodak gel filters and holders, and ....... so much stuff. But I digress 🙂