The whole Contax II/III and later IIa/IIIa's and the pre and post war 35mm lenses is a bit of a saga. I kinda gave up. If I do end up with a 35mm lens, it may be a newish Voigt or a Nikkor. I even understand there was a post war Jena 35mm that fit the IIa/IIIa cameras. Bio...? -tar maybe.
Well the Planar fits the IIa/IIIa - said to be a great lens too. I have a pre-war Biometar that fits all Contax. Neither of these lenses have a deep rear element - that's why.