My impression is that it was more a matter of material suppliers issue between pre and post war. Most of the manufacturers and suppliers changed their production significantly during the war years to meet the obvious department of defense needs. After the war, a lot of them never returned to the old model, so manufacturers used what was available.
Look at Kay as an example. They were never sophisticated or well designed, they were made to be first and foremost affordable to the masses by a huge manufacturer. The early models had that horrible almost oak looking outside veneers. By the early '40s, the veneers and the composition changed. Post WW2, they were using maple for the inside plys: 1945-49 are my favorites as a technician because the maple stays together and is easy to work on. They also seem to have slightly more attack to the voice, rather than the late '30s thud. I believe there was a management change in approx. 1951, and also a switch to that brown and green tulip poplar for the inside plys on both the top and bottom; an awful choice that falls apart as a miserable mess when you try to open the box, and drinks up hot hide glue like it has been in the desert for 100 years. The post 1951 bassbars seem to separate at a much higher rate, not the bassbar from the outermost ply, but rather the bassbar with two feet of the actual ply layer still connected rip off en mass. 'Pretty sure there was another management change in the late 1950s along with some ply composition changes that lasted until the end of production. Top stiffness was mainly a matter of the quality of the laminates- I've seen many that were built within a few months of each other where one is very stiff with no deflection and the other is a floppy mess. Once they started making the S9s and the Chubby Jackson 5 strings, there seemed to be more bodies that were built much heavier, but that may have been an attempt to use up lesser grade materials. A lot of people get worked up over particular years. I've played plenty of pre war models that were awful sounding and some surprising late 1960s that were supposedly less desirable and half the price that sounded great. I mainly only buy and sell the pre and post war models for the simple reason that I don't have to listen to all the internet wankers whining about those years....
The pre-war American Standards are lighter built than the 1950/60s. They took a longer break during the war years. The similarities are enough that I'm sure they were using common suppliers and communicated with each other often. Remember that none of these were heirloom investments; they were cheap student instruments intended to fill the gap initially created by the late 1930s German embargo leading up to the war. For $230, you could buy a new Kay at your local music store and go. If you liked it, play the old Kay until you saved up $400 and you could buy a 200 year old Italian bass back then and throw the Kay in the bonfire.
I've heard that Kay learned the Germans were not going to be at the 1937 NAMM show, so they outsourced a generic bass from available parts with the idea that it would look good at the booth and maybe they'd sell a few dozen. They took orders for ten times that amount and then went home and started making them with what they had available, not as a designed product. Hence the oversize guitar dovetail, because they already had an in house jig and the factory was making something like 100,000 instruments a year already....
Old ply basses are just a part of my business; I don't get too worked up about the subtleties, more so the structure and setup. When a 200 year old Italian bass or Lloyd Loar signed mandolin or a pre war Martin or D'Angelico come into the shop, I spend a lot of time measuring all the details- I've made hundreds of blueprints on those!
Beautiful!!!