You already know the answer to this......There are four individuals currently being prosecuted by ATF for a variation of this type of proposition: transferring serial numbers from registered weapons that were very cheap to newly made weapons that had much higher values. Transferring a registered serial number to an unregistered MG is "manufacturing" a 'new MG', which was prohibited by law on May 19, 1986. Not to mention the issue of removing a serial number, which is a felony, among other equally felonious issues.
ATF would have great difficulty discovering such a transposition, but with the help of Murphy, it is entirely within reasonable probabilities. The discovery of the activites of the four defendants attests to Murphy's expertise.