If my memory serves me, the lens you have is a pre war lens and won't fit post war bodies. Or it is a Russian fake. Zeiss made some lenses that would fit your IIa. The Planar for sure, and I think there is another. I use to have a Biometar made in East Germany that was a good lens, but I guess they are rare and cost a few bucks now. I probably shouldn't have sold it. It might be cheaper to buy a II pre war body than to replace your lens. Every lens fits the pre war bodies.
By the way, the Russian 35mm won't fit the IIa either I don't think, which is a shame because it is a good lens. Again, going off memory from a long time ago....
Cosina made a modern Voigtlander 35mm Skopar that would fit your Contax. Might be hard to find one though. Cameraquest was selling them cheap a few years ago.
I love Contaxes. Back in the 90s I had six or so bodies. I wish I never sold them all off. I only have a IIIa now that is all beat to heck and a pre war 1.5 Sonnar that I couldn't sell because it is one of the finest lenses I have ever had! Zeiss knew what they were doing.... If the Contaxes had a wind lever and framelines Leica would not have survived.
Hope that helps you.
