Filter size for Schneider 50mm f1.5 Xenon

[herb]

This indeed an old one, Schneider Kreuznach sn Nr2110460. A 43mm is too small, and a 46 ever so slightly too big. Was there some kind of odd system??

 

[paul_c5x4]

Sounds like it might be a Series VI filter which I believe is about 44mm