The 51S-1 is the result of the military's desire to have a continuous tune receiver in the S-Line package. These receivers were seen mostly in rack mount configuration. The Air Force bought these units to support SAC HF communication applications worldwide. The receiver came in numerous configurations and was updated for specific applications. The versions are 51S-1/1A/1F/1AF/1B.  The 51S-1 receives USB, LSB, AM, and CW signals continuously over the frequency range of 0.2 to 30 MHZ in thirty 1-megahertz bands. The model 51S-1 can operate on 50-400HZ AC at 110VAC to 220VAC.  The 51S-1A uses a 28-volt DC solid-state power supply.  The 51S-1B has a rear-mounted junction box that provides connectors for power, control, audio, and antenna lines for rack mount airborne applications. The unit pictured here is a early model and has not been restored.