It is a Second World War vintage transmitter. Used mostly in heavy aircraft. It was also used after the war by civilian airlines. It was made by Collins Radio. The navy called it the ATC and the airforce called it the AN/ART-13. Google both labels and you'll find a ton of stuff. You can download the manual from BAMA here:
http://bama.sbc.edu/military.htm
Still in use by some ham radio operators. The autotune mechanism is neat.
Bruce