Look Ma, No Hands!
I have a theory about Whizzer serial numbers, but it doesn't mean that it is true! I have 5 factory Whizzers. Three of them which I believe are all 1948's have a prewar serial number. Another one has the early flat crimps with a standard C serial number (I think that one is a late 1947 or early 1948). The last one has a 1948 serial number that places it just days before the fire. All bikes have S-2's with no knurls. I think Schwinn used left over prewar bottom brackets for their new line of 1948 WZ507 bikes. Maybe this changed after the fire? Besides the dimpled chain-stays, and welded on brake clamp, these odd serial numbers could help separate the WZ507 models. Anyway that is my theory! I am sure their are allot of guys out there with better info.
I like the Theory! You having actual bikes helps w/ eliminating a lot of speculation others may bring to the table. It also makes sense that Schwinn would be trying to use any extra stock parts they had.. The fire makes it difficult to pin down exact dates based on sequential serial numbers, but it does make for a more interesting mystery if you have one of these bikes! Awesome reply!