The 75S-3C Receiver provides extended range coverage
over the 75S-3B. It is identical to the 75S-3B except that an additional
HF crystal board and front panel selector switch are included. The
board for additional crystals is located on the top of the chassis and the
standard group of amateur band crystals is mounted in a board on the underside
of the chassis.
Information from Collins Amateur
Radio Equipment Catalog No. 32