The ATF Form 6 would be available from ATFE - that is for the importation of any weapon or destructive device. As for the "Tax Stamp" for possessing such weapons, I hope that is considered not releasable. ATFE is required to keep those records (forever), but its release would jeopardized the privacy of personal information for all lawful owners of all NFA weapons. (NFA encompassing machine-guns, short barreled shotguns/carbines, destructive devices, etc.) The bottom line is, if someone possesses a destructive device, they must have the federal paperwork immediately available for presentation to law enforcement officers.
Search google for various combinations of: DEWAT, ATF, register, and destructive device