Anti-Aircraft Artillery Range Indicator, US Navy, Mark 1, 1944
Here is a WWII USNavy Range Indicator, Antiaircraft Gunnery, Mark 1. It is a hand held finder used to ascertain the range of an enemy aircraft based upon its known wingspan. They were only as accurate as the observers knowledge of aircraft identification and of course, how rapidly he could spot, identify, track and calculate the information to pass this on to the gunners (usually manning 20mm to 40mm guns or a 3" gun mount typically used on auxiliary ships and small craft. i.e. PT boats, LCT/LCI, PC's, etc). The observer held the indicator about two feet from his eye, measured the wingspan of the aircraft between the two wires in the center viewfinder, which widened or narrowed as the interior metal ring is rotated, and then read off the estimated range to the gun captain.
Date of manufacturer is noted as 1944. Note the Italian SM 579 Torpedo Bomber & the FW 187 that was never operationally deployed yet identified by US military intelligence as 'in production'. These aircraft are described in detail in training manuals used to prepare for the TORCH landings.
The problem with using this indicator was not only identifying the aircraft, but measuring the wingspan as the aircraft changed its angles of approach. Some enemy aircraft were easily identified due to their distinctive shape and engine placement, while others were more difficult to ID, especially when moving 300 to 400 MPH & the observer was standing upon a pitching deck with guns banging all around. Still, better than having no aids at all.
Thanks to Salvage Sailor who posted the above description on the US Militaria Forum.