You can request ATF to issue a new serial number under the circumstances. I bought a Colt Model 639 (XM177-E2), unfired in the box with matching serial number flash/sound suppressor. Colt sold only 100 of these to a non-military purchaser "Aeromarine" who provided Colt with export paperwork. Aeromarine then never completed the export - I bet by choice just to obtain the weapons? Who knows, maybe their export deal fell through and they were left with 100 highly-desirable weapons legal for US individuals to own. Anyway, my seller told me he traded Heinrich Himmler's personal banner for it. The serial number on the suppressor has a story. After Colt designed the flash suppressor, ATF ruled it to be a sound suppressor as it reduced muzzle blast by 3 decibels. Owners of the 100 machineguns were sent a letter by ATF to turn in the suppressors. Most buyers did, but a handful like my seller refused numerous requests by ATF on the basis it was not a "sound" suppressor when he purchased it. ATF apparently realized their request to turn it in was a violation of the ex-post facto clause of the Constitution; i.e., you can't turn an action legal when undertaken into illegal. Same as the amnesty allowed when the law changed in 1934 with old machineguns purchased legally prior to the NFA of 1934. Finally ATF relented if you submitted the "offending" suppressor for numbering. Unless requested, ATF stamped "ATF xxxx" but my seller insisted on a matching number with the weapon. My gun's ATF stamped/etched serial number was noticeably shiny against the parkerized original finish. So I think you have a basis for a new serial number, but it would likely take some time and hassle to do so.