Sorry for the necro, but I have been playing with my 122 key Model M terminal board and have some info that may be relevant:
Firstly, the Caps Lock key fits the right shift barrel (both the right size and stem is in the right position) and looks just like that stepped shift, just with the wrong label.
Secondly, the 122 key Model M (at least my 1394312) has the barrels AND membrane elements for all the positions of the 127-key IBM 4980 terminal keyboard that ch_123 shows the picture for, so all the positions can be used. I haven't checked the scancodes sent, but I'm replacing the controller on mine anyway.
I'm converting mine to 60% and will probably go with a standard ANSI layout, but with small left shift (like the ISO layout) and extra key to the left of Z. I'll use that for `~. In the meantime I have 1x keycaps on all the barrels and will wire up my Teensy controller to be able to use all the positions, so I can alter the layout whenever I like.
With this in mind, I would also like to get ahold of a 1.75x right shift key. Did you ever come right with this, TheRavenDog?