A lot of old cine prime lenses have Series filter threads. Series 4.5 for the tiny lenses for 8 mm and Series 5.5 for lenses for 16 mm are typical. Here's a guy who supplies an adapter from Series 5.5 to 49 mm.

As I mentioned before, I used the universal mix-and-match BPM bellows and this was long enough ago for BPM parts to be readily available and cheap. Here is one on eBay.

Best,
Helen