I think you have one of the "C" lenses -- with Compur shutter? If so, I think they take a "Bay(onet) 50" filter. Many manufacturers make "Bay50" to - probably 67mm would be appropriate - adapters, enabling the use of screw-in filters.

I have one made by Hoya; I'm sure Tiffen and B&W make them as well.

Check the Tiffen web site - I think it is Tiffen.com.

Also ... Cokin provides a "Bay50" - "P" (I would strongly suggest the "P" series) adapter ring. I use those as well. They have the advantage of a built-in (?) - auxilliary(?) lens shade.