As I understand it, the black J-12s are more likely to be the later models, and hence, may be a more modern lens. The first two digits in the serial number should indicate year of manufacture. I don't have experience with a lens for a Fed, but I have had a 1987 black lens in LTM mount for a few months, and I am very happy with it. Look at the recent aircraft photos in my gallery to see examples of this lens, used on a Leica IIIf, with Fuji 200 film.
Jim N.