The "original" Model 1950's and 1955's (Pre-Model 26 and Pre-Model 25) would have had the serial numbers on the butt, on the frame under the crane, inside the barrel shroud, rear of the cylinder, under the extractor star, and visible when disassembled, under the sideplate as well.
Around 1957 with the introduction of the model numbers, the Model 26/1950 and the 25/1955 and 25-2/1955 were only marked on the butt, frame under the crane and under the extractor star. And I think the sideplate was still matched, with either the S/N or assembly number.
I have a Model 1950 pre-Model 26 that shipped in April 1956 which is marked inside the ejector rod shroud and a Model 25-2 with the mis-marked MODEL 1950 barrel which only has the serial numbers in the post-1957 areas.
Vettepartz gun with a serial number in the shroud is a puzzler.