An S stamp under the crane or on the crane is not unknown, my early 70s Model 66 has the same font on the inside of the crane, but not on the frame. I wish I knew what it stood for.
A small S&W trademark was used on the left side of early guns, all the sideplate pics I have seen use the large trademark, so you may be right, it may be a poor copy of a bubbaed wartime S&W.
EnderWiggin:
tpelle is correct about it being mechanically suspect, I had a small J frame .357 magnum that I replaced the barrel on with a custom four inch barrel with no locking lug. 5 rounds of Plus P .38 special sprung the crane. Don't shoot PlusP in this gun.