I doubt it is a factory rebuild as it would not have left the factory in that condition.
I have a S&W M1917 without a number stamped on its butt which I assume was built part by part as some parts are not finished while other ones are. Lunch box specials are not common but they do occur. Getting the receiver, barrel and other parts BEFORE a number would have been stamped on it would ensure that the theft would not be discovered by any of the company inventory checks. No one would miss one part missing from a big box of newly manufactured parts.
An employee wanted a gun and figured he could smuggle out the individual parts and 'make' his own gun at home.
