About the only reason I can think of that the focus is off on the III, where it comes to rest at a different spot, is that the RF Cam of the J-3 is at an angle. I've had this happen before when the helicals have a lot of slop on them, and require heavy grease to fill them in. The helical with the RF cam can move at a slight angle, messing things up. OR- the threads for the helical to screw into the lens mount (portion with the 39mm threads) is at a slight angle. 0.1mm will mess things up quite a bit.
I am "the guy Here" that wrote the J-3 article on Kim's web site. I've worked on at least 30 of them, and YES quality control on the machining is all over the place. But a good J-3, shimmed for the Leica, can be the equal of a Nikkor 5cm F1.4 or Zeiss 50mm F1.5: