I'm on the phone with Quenton Guenther (one of the top Whizzer gurus) as I type this, and by the serial number you have a late 1946 or 47 "H" model engine. The controls are from the later "J" engine. The rest of the bike has a mix of later year features, such as the "easy glide" clutch arm and the tank graphic is a design that was used 1951-on. The rack is a later design but the frame looks like the correct Schwinn S-4 (made for the Whizzer engine). The engine is all correct, but the aftermarket exhaust is "Model 52" twin pipe made by Richards Cycle Company of Chicago, which came out in 1952. Everything is all correct, but not specific to one particular year.
All in all he says you have an extremely nice bike!