These are the fiber boards needed to make one side loading single coil - almost identical to a standard single coil, with a base, housing 2 eyelets and the threaded height adjustment holes, and a smaller top section.

Not all that common, but they occasionally turn up on guitars with very neat, close routing on the front face, without a pickguard. They also occasionally turn up as "specific" bridge pickups on the Stratocaster®, whre the smaller profile on the base means they're a little more forgiving to fit.

Eyelets Supplied, but not fitted.