The main difference is in the length of the upper legs. The upper leg length has to be matched to the length of the head tube. Prewar Schwinn boys bikes had short head tubes, 5". Post war boys and girls are the same, 5 1/2" and interchangeable. Also, the truss rods are 1/2" shorter on a prewar boys springer.
In '55 the style changed completely to the style used now by most aftermarket companies.
Now, to make it even more confusing. Prewar girls bikes had different head tube lengths, depending on the year. A prewar girls springer from a frame with a 5 1/2" headtube will work on post war bikes. This was a one year only, I think '41. Other years of prewar girls head tubes are all over the place on length and springers will only work on the specific frame they were made for.
On the prewar boys springers, there are two styles. (not counting locking, non locking, etc.) The earlier style went from '37 ? (first year of springer) to around '40. The upper legs have a strong curve to them. The '40-'41 style springer legs look more like the post war, they are straighter. Gary