SCR-587 (left) and Navy ARC-1 (right) range "B" tuning units
Essentially identical to the AN/APR-4, with only minor differences like a variable IF gain control versus discreet calibrated steps, the AN/APR-1 was most often used by Navy patrol aircraft. It formed a bridge between the Signal Corps SCR-587 and Navy ARC-1 with roughly the same capability. A comparison of two of the early tuning units is shown below. By the way, these early tuning units also work in the last of the line, the AN/APR-4Y, a component of the AN/ALR-5 system.
Later tuning units (with -B suffixes like the one in the APR-1 photo below) added a much needed scanning motor to sweep between two preset frequencies. Trying to tune back and forth by hand probably led to carpal tunnel syndrome in short order.
