Daimon
Established
In my case these are 1965 (silver) and 1987 (black) J-12s. None of them have collars around rear element (I think at least 
) - elements are coated, hard to tell. I'll take some photos but in a week earliest.
1987 was focusing properly (and had quite thick shim). 1965 had ultra-thin shim and was completely out of focus. Made shim thicker and now it focuses properly on both 1m and infinity. RF coupling at infinity is also very close to other lenses.
Both work fine on Leotax F, none will mount Canon P. On FED-2 silver one has problems. I haven't checked black because silver one was supposed to be the one for FED
Anyway from what I've seen it's not the "bottom" of rear lense but side of it that comes into contact with camera RF sensor
) - elements are coated, hard to tell. I'll take some photos but in a week earliest.1987 was focusing properly (and had quite thick shim). 1965 had ultra-thin shim and was completely out of focus. Made shim thicker and now it focuses properly on both 1m and infinity. RF coupling at infinity is also very close to other lenses.
Both work fine on Leotax F, none will mount Canon P. On FED-2 silver one has problems. I haven't checked black because silver one was supposed to be the one for FED
Anyway from what I've seen it's not the "bottom" of rear lense but side of it that comes into contact with camera RF sensor
Daimon
Established
Ok so finally could take photos. So diameter of glass element is 29.95mm on my copy:
Here's how it looks on minimal distance and infinity:
It screws into camera but when lense is focusing close to minimal distance (i.e. max rangefinder arm movement into the lense side) i can feel and see (rangefinder image "skips") when focusing. I don't thing there's anything particularly odd about rangefinder arm - it probably just lacks tiny clearence
Here's how it looks on minimal distance and infinity:
It screws into camera but when lense is focusing close to minimal distance (i.e. max rangefinder arm movement into the lense side) i can feel and see (rangefinder image "skips") when focusing. I don't thing there's anything particularly odd about rangefinder arm - it probably just lacks tiny clearence
Check that the screw holding the arm is tight- to rule out play in the arm.