This is a 1940's Doran Cunningham air horn. Usually found on WWII US Navy destroyer ships, and also found on some older ferries. There are only 2 of these that are being used that I know of. Mine and Adam's (Airraidsiren on YouTube). Very rare that only six have been seen in pictures and each one had a different configuration. The 6B is close to 200lbs. of brass, has a 12" diaphragm,my 6B measures 36 1/4" long, and has an 18" bell/horn opening.
There are markings at the footing of the air inlet "DD416". This is not the biggest horn made; but is considered the best because of it's sound!