Optics Theory -This forum is aimed towards the TECHNICAL side of photographic OPTICS THEORY. There will be some overlap by camera/manufacturer, but this forum is for the heavy duty tech discussions. This is NOT the place to discuss a specific lens or lens line, do that in the appropriate forum. This is the forum to discuss optics or lenses in general, to learn about the tech behind the lenses and images. IF you have a question about a specific lens, post it in the forum about that type of camera, NOT HERE.
If you're just wanting to transplant the rear optical block from the Contact to the LTM lens, it's absolutely possible. You'll want to ensure that the J12 lenses are both from relatively similar build periods. For example, I wouldn't want to do this transplant from an early 80s lens to a 1950s lens but even then it can definitely be done. Considering the QC that went into most of the FSU gear, you may wind up with a better optic than the original.