Just returned home and located my 120 AM.
First image shows the lens split with front part incl. the Copal shutter, rear section, BR2 reversal ring, 37-52 step ring (for centering), a T2-M39Leica adapter (distance spacer). Also apparent is that the lens indeed is multicoated (we discussed this aspect earlier in this thread).
Next, put the 37-52 onto the rear end of the shutter with the narrow opening down. This will to the required first approximation center the rear lens. Add the T2-M39 spacer, put the BR2 on top, and stick the rear lens into the opening and screw it home on its threads at the back of the shutter unit. That's it.
This trick works because the rear lens is a trifle wider in diameter than the free opening on the F-mount bayonet. Were the lens to be mounted directly on an F-mount camera, this of course wouldn't work as the rear element would jam in the camera throat. However, due to the design of the 120AM, this mounting approach is impossible as the lens needs much longer extension just in order to focus to infinity (for which, might I add, it is not designed ...). All bellows or extension tubes have the required internal clearance.
Assembly time about 20 sec with practice.
Other solutions are of course feasible, but one uses what is available on the work table for such minor modifications. I have used the lens thus modified for decades. I add a narrow extension with a CPU inside to the bellows to have the correct EXIF data recorded as well.
