Radio Altitude Indicator (Altimeter), ID-14/ARN-1 for Radio Set AN/APN-1 S2 Tracker after which it is only

after which it is only necessary for him to "range and track" the enemy plane

which would release the lens for replacement

The K-13 Compensating Gunsight

made for the Bureau of Aeronautics of the US Navy by Elgin (part number 2137

The manual is in very good condition despite some discoloration on the cover and some minor deterioration on the upper corner of the last 4 pages

Radio Altitude Indicator (Altimeter), ID-14/ARN-1 for Radio Set AN/APN-1 S2 Tracker after which it is onlyThe WWII era Radio Altitude Indicator, ID 14 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. Made by Roller Smith. See image of technical description of the system. 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