Various Manufacturers Rear Derailleur Adaptor Claw
Adaptor claw
to attach a standard derailer to a rear dropout that does not have a built-in Derailleur mount.
The bolt and its special shoulder nut fit in the back of the axle slot. When normally installed, the left end of the wheel's axle will wind up near the middle of its axle slot.
This is normal. There are millions of bikes on the road using this system.
