The following statement [often heard in the real world] is ONLY true when it applies to Schwinn built bikes; all others brands changed @ latter times post war... Pre War Schwinn were all REAR DROP OUT; I believe this is true including 1942. Every Post War Schwinn was a FRONT DROP. [I'll qualify by saying I have no knowledge about the skinny tired road bikes like 'the World' models]. In 1945 Schwinn fired back up; but, I've only seen catalogs with the 'road' bikes [skinny tires]. In 1946 they put out a fine line-up adding the ballooners we all rave about. Take your serial number and go to the top thread of this Schwinn forum and there is a thread about Schwinn Serial Numbers... open and scroll down... maybe 17 pages ? is a green sheet [photo-copy of pre-war serial number ranges... FIND yours!
Onumbers-Znumbers comes out in 1938...