VL,
Oh, crap, look at this.
Original "registered" .357s could apparently be custom ordered with virtually any barrel length between 3.5 and 8.75 inches.
The pre-war unregistered Magnum, made from 1938 to 1941, and the post war guns could be had with 3.5, 5, 6, 6.5, and 8 3/8 inch barrels.
In other words, the traditional barrel lengths have been available form almost the get-go.