Zeiss ContaxForum for the classic Zeiss Contax I, II, III, IIa, IIIa , G series, and if you want to push it, the nice Contax point and shoots. Some spill over from the Kievs, the Soviet copy of the Contax II/III can also be expected. Plus the ONLY production camera ever made in classic Zeiss Contax Rangefinder mount WITH TTL metering ... the Voigtlander Bessa R2C.
I think a filter or a step-up ring is the only solution you can use without getting vignetting on this lens. I found a B+W UV filter which is silver on the outside and black on the inside of the ring, looks quite nice too.