Nice. I've had a couple of good J-12's over the years. The best was a mid 50's one for Kiev (gee, surprise there!) but I had good LTM ones too. I should budget one in for my Canon 7 someday (I had a black one that fit in the past).
On mine- three set screws at the top of the focus ring. Looks like they can be loosened and the ring be turned. Next time I setup for lens work, I'll try it- mine is slightly off. Right now my "electronics lab" is at home, and has over-run my workshop used for camera work. No complaints- I get paid, no commute, and get to program in DOS and Fortran on a custom designed computer.