Serial numbering was originally instituted by the gun companies, specifically Colt, as a means of tracking production changes and as part of warranty service. It was not until the 20th century that governments hit on the idea of registering guns as a means of controlling political opposition (usually touted as controlling crime, even when the "crime" was voting the wrong way).

Since serial numbering and the associated record keeping cost money, most manufacturere did not number their inexpensive long guns prior to 1968. After the passage of GCAicon '68 many makers discontinued their cheaper lines of shotguns and .22 rifles since they felt that the additional cost would be more than the traffic would bear. Higher quality guns (like the Winchester 70) were always serial numbered.

IIRC, the original FFA, required serial numbers on handguns, but it might have just been in response to growing state registration laws.

In any case, those guns made before the law required serial numbers were "grandfathered" and are perfectly legal to own, though it is illegal to remove a serial number or to possess a gun from which the serial number has been removed or defaced.

Dealers handling guns that never had serial numbers simply note that fact in their bound book and any other documentation.

Jim