The underlying question is how ZIP codes in Chicago and other large cities were assigned. For the most part, most Dopers would know, ZIP codes in large cities just added the sectional center prefix (606) in front of the zone numbers (40, for example) that had been introduced during World War II because so many experienced postal workers had gone into the service.
But how were the original zone numbers assigned in 1943? My theory is that they were based on the sorting cases already in use. As a city like Chicago grew, additional branch post offices and carrier stations were established. The arrangement of slots in the sorting case used for sending letters to the other branch offices would have been standardized among offices and infrequently changed, as that would slow for weeks the work of trained clerks who already knew the scheme.
In Chicago, the lowest numbers are for the downtown area, followed mostly by areas where branches would have existed prior to 1900. The larger numbers look like they could well have been added later, as new branches were established, and those new offices were simply assigned slots at the end of the scheme/bottom of the case. Thus, when zone numbers were introduced, they simply numbered the slots in order.
(In suburban and rural areas where each town has its own ZIP code, it looks like the last two digits were assigned in rough alphabetical order, though there were many exceptions as well as later additions and splits.)
I don’t suppose anyone is around who can speak with authority about how they did it in 1943, but does my theory hold water with folks who’ve thrown a scheme?