Radio Altitude Indicator (Altimeter), ID-236/APA-61 for AN/APA-61

The WWII-era Radio Altitude Indicator, ID-236/APA-61 was a component of Radar Bombing Navigational Computer AN/APA-61.  The indicator was used to measure and indicate to the pilot the altitude over the terrain during flight.  Made by Roller-Smith. This unit looks to be originally of the AN/APN-1 system and was modified for use in the AN/APA-61 system.  The nature of the modification is unknown. The precursor AN/APN-1 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, 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 194