Cool items! The mystery part is also Mustang related - it is an antenna wire guide/pulley that was mounted to the canopies of P-51D/K's. The beacon receiver antenna wire passed through the canopy, through this clear fairing (with a pulley inside), and back to the tail. In most cases these antenna wires were removed once the aircraft was in its Theatre of operation, as there was no longer a use for the beacon receivers outside of the U.S. The pulley/guide fairings were also then removed and often an oval-like flat piece of plexi or other material was screwed into place over the hole in the canopy.

Photo by Glenn Wegman:

