The missing screw is probably an M5x50mm, the lug on the motor has an internal thread. ie no nut needed (in theory) It seems that these work loose, leaving the motor to vibrate tus cracking the adjustment bracket.
I'm just about to fit bracket no.2 after almost 2yrs.
I had intended to check the tightness of the M5 screw, but didn't.
Even though I used a spring washer the thing obviously worked free.
Peter
November 2008