Long guns ser# 'requirement' done by their mfg'r goes back to at least to the
FFA38 (Fed Firearms Act 1938),,if not the NFA '34.
FFA38 has language that states Removal, alteration, obliteration of a mfg'rs applied ser# is a felony,,ect.
The same language was rolled over into GCA68 when it took the place of the 1938 law.
I don't know that FFA38 specifys which firearms catagorys would be ser#'d upon mfgr (CF Rifles and all Handguns),,and which were optional (Shotguns and rf Long guns).
The GCA68 does however specificly demand that the last 2 classifications now join the cf Rifles and all Handguns in being ser#'d at the time of mfg.
All imports shall be ser#'d as well.
This simple gathering together of all classifications of firearms and making them all required to be ser#'d at the time of mfg has led to the misconception that it was at the time of the passing of GCA68 that all classes were FIRST required to be numbered.
When actually it was only the shotguns & rf long guns that were in that group.
The CF rifles and all handguns had already long been required to be ser#'d.
Certainly centerfire rifles were required to be ser#'d prior to the enactment of GCA68.
There are several instances prior to GCA68 where US gun mfg's got into hot water with the IRS (the pre-BATF Fed Firearms Enforcement Division at the time) over NOT ser#'g center fire production long guns being produced.
Marlin is probably the prime example often brought up.
Their centerfire Levermatics, the first 5000+ out the door unser#'d in 1964/65.
Savage also was wrist slapped for the same type of conduct. I believe it was over the Model 219 break open single shot rifle.
GCA68 ser# requirements added that a date or date code was somehow included in the ser#.
The 1986 Crime Act added another requirement that the ser# include an Alpha character(?) identifier to show the specific model of the firearm. This to separate Different models of same caliber.....Lots of S&W 38's around.