filter sizes

I need some ND and 81a filters for some bolex lenses

5.5 pizar
5.5 switar
13mm pizar
13mm yvar

and others as well..

one of the lenses had a series C converter on it. Central camera says there's no such thing as series c

now what series are these little filters? are these the roman numberal sets? Most importantly: where can I find these things?

Have a look here: (there was a url link here which no longer exists)