> So, will a post war Sonnar f1.5's elements fit into a J-3 ltm mount?
NO! The post war lenses will not work. The optics and fixtures were enlarged. The Zeiss Opton is much different construction, and the post-war Zeiss Jena's are larger diameter optics and threads.
The Pre-War and Wartime optics modules will work. The fixtures are Longer than the J-3, and the "prescription" of the glass is different. BUT- in the worst case, take off the variable stand-off ring of the J-3, polish it down, and use it on the Zeiss module to go into the Jupiter focussing mount.
The 5cm F2 requires that you use the J-8 aperture ring in place of the Zeiss ring. The 5cm F1.5 aperture ring can stay in place on the J-3 Mount. I've done 5 exchanges to date.
The J-8 Module WILL NOT go into the collapsible 5cm F2 mount, but an I-61 module fits after polishing down the aperture ring. The J-3 module fits into the Contax 5cm f1.5 mount, but you need to polish it down a bit. The J-3 mount is shorter.
Yup- That Canon P would look something like this...