That Schwinn serial is pretty interesting. Looks like it's machine stamped, but different than the 1937 machine stamped numbers and it's missing the prefix letter. If that is hand stamped, it's the most perfect looking hand stamped number I've ever seen. Lined up even, equally spaced, straight and evenly struck. Possibly one of the first ones stamped when Schwinn was setting up the new stamping machine? 36-37?