Interord, Century, Sarco and all those folks are keeping records when you order parts kits. That is why a lot of people, especially after that first letter, call or even visit from the ATF about a parts kit will only buy their parts kits cash over the counter at shops or shows.
I sell parts kits at gun shows from time to time, and regularly get guys telling me that I am running ripoff prices at an average of $40 markup per parts kits. The thing is, as soon as the very true stories get around about how the ATF suddenly makes up reasons about why they want to "recover" parts kits from the likes of Coles (the infamous uzi kits), Century (fal kits), Internord (PPSh-41 kits) and Global Sales (sten "spring break" kits), these guys don'w have much of a problem buying kits where no records are being kept.
As for proving the parts, a lot of the parts kits have barrels and other parts that are also marked with the serial number of the gun, and believe it or not, that number is sometimes recorded if not on the individual buyer, then by the batch when the importer gets them, or even by the exporter when they sell them to the importer.
What you are obligated to have a serial number on is the receiver. Serial numbers on other parts? well, sometimes it is best if they go away.