"WW2 fit" is basically the best you could do when picking from "Regular" sizes in a few different contracts at most. Getting what to anyone in particular might be a "perfect" fit would be random, or you'd have to tailor it (likely only done for people grossly out of size who had the wherewithal to do so).
Given that even within manufacturers and contracts actual sizes varied within a "Regular" size, a "real" fit would be within whatever the range of variation was within a mfg/contract. If a size 42 Dubow 20960 had a nominal set of measurements that varied by ~ +-5% for each (chest, shoulders, etc) then if you wear a 42, and your jacket is a 42R +-5% then it's a "ww2 fit." If your custom 42 is off by 15% in some measurement, then it likely isn't strictly speaking an "issue" fit, but would be a ww2 fit for a private purchase or re-tailor job (both of which happened I assume, even if rare). It's also important to note that the difference to the next size is in fact small. A 40 to 42 is a ~5% change, so even such a small individual variation in jackets can really alter things in terms of what "size" they are.
Bottom line is that the real life variation was so huge as to make a notion of a particular fit pretty unlikely.
Given that even within manufacturers and contracts actual sizes varied within a "Regular" size, a "real" fit would be within whatever the range of variation was within a mfg/contract. If a size 42 Dubow 20960 had a nominal set of measurements that varied by ~ +-5% for each (chest, shoulders, etc) then if you wear a 42, and your jacket is a 42R +-5% then it's a "ww2 fit." If your custom 42 is off by 15% in some measurement, then it likely isn't strictly speaking an "issue" fit, but would be a ww2 fit for a private purchase or re-tailor job (both of which happened I assume, even if rare). It's also important to note that the difference to the next size is in fact small. A 40 to 42 is a ~5% change, so even such a small individual variation in jackets can really alter things in terms of what "size" they are.
Bottom line is that the real life variation was so huge as to make a notion of a particular fit pretty unlikely.