It's beyond the original scope of the thread, but it is a good point.

When I took my M-II derailleur apart to take photos, it was the first time for me. As I disassembled it, I really wasn't aware of the little tab and could easily have lost it if I was working on a messy workbench.


Here's a shot of the derailleur after removing the allen nut on the lower pivot, where you can see the tab installed in the slot in the pivot bolt.....




in a later photo, I edited the photo to specifically identify the little tab and say where it fit.....




For reference, the tab is installed with the small "wings" at the bottom and the narrow part of the taper at the top. My guess is that the wings keep it from creeping out of the slot and escaping.

Since I like to live dangerously, I'll also guess that the purpose of the tab is to keep the two sides of the split bolt from bending inwards when the bolt is tightened, which would prevent the nut from being tightened.
I haven't taken the time to test this theory, though.

All in all, it's certainly a more compact design than the first generation Cyclone, and possibly lighter, but somewhat more complex and with more small bits to lose.


Steve in Peoria