Originally Posted by ClydeClydeson

The flat part with two holes is almost definitely the standard bolt pattern for a reflector and for many lights, and as mentioned above, probably will be a perfect mate to the built-in reflector bracket on many racks and other accessories such as fenders. The serrated face with a single hole is to mate with another part with a similar serrated pattern, either on a light or on a bracket that clamps to the seatpost or frame.