I would have to see a good, detailed photos of the questioned revolver before I would even consider it being a 1917, modified or not! I would bet that it was mis-identified by the LGS or other entity that has it for sale. The only other reasonable explanation is that it is a military 1917 converted to .38 Special by replacing the barrel and cylinder with those parts from a
.38-44 Heavy Duty. If that is the case bottom of the barrel definitely would not be marked United States Property as the 1917s were.

It is no trick to re-stamp an un-numbered replacement barrel and cylinder so the numbers match the frame. It is highly un-likely that the font would properly match the font that S&W would have used for those parts if this is the case.

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Gunsmithing since 1961