Radio Altimeter Altitude Limit Selector Switch, SA-1/ARN-1 for AN/APN-1

The WWII-era Radio Altimeter Altitude Limit Selector Switch, SA-1/ARN-1 was a component of Radio Set AN/APN-1, used to measure and indicate to the pilot the altitude over the terrain during flight.  See image of spec sheet. The Radio Altimeter was used in a number of aircraft of the US Navy where close monitoring of height over terrain was critical, such as torpedo bomber TBM Avenger (see image from TBM Flight Manual), dive bomber SB2C Helldiver, patrol aircraft PV-1 Ventura, PV-2 Harpoon, PBY Catalina, PBM-5 Mariner, and PB4Y Privateer.  It was also common in aircraft of the US Army Air Force such as the C-47 Skytrain transport, as well as Korean War-era aircraft such as the A-1 Skyraider. According to the description contained in the Graphic Survey of Radar and Radio Equipment, Section Four, Radar Navigation Equipment, dated 1945 (see document under Downloads):  "Radio Set *AN/APN-1 is an airborne frequency modulated radar altimeter designed for installation in aircraft to provide di