Originally Posted by
artclone
(This is for future interest. OP has already made a solution.)
The nut locks the bolt instead of on normal derailleurs, which rely on tightening all the way down to secure everything.
The derailleur is not suppossed to be tightened down and made immobile, but it will work. The only disadvantage is you lose the ability to swing the derailleur back when removing a wheel. During riding it is locked in the forward position anyway.
Here's the nut and you can see the two washers:
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Ouch. Notice that the derailleur hanger has been filed/ground down to half thickness? That's one way to get clearance. I think I'd rather mod the derailleur than the frame.