Prior to the GCA68, shotguns and cal 22rf Long guns were not required to be ser#'d by their mfg'r.
The mfg'r could ser# them if they wished to do so,,but it was not a Fed requirement.
Winchester chose to ser# some of their 22 rifle production (the higher priced guns) but not others.
Winchesters Model 21's are all #'d, as were M42, 52, 63, 61 and others.
Some of the simpler 22 rifles like the single shot bolts and others were not #'d
Savage/Stevens the same way. Many of their less expensive and simpler design guns were not #'s. Others were ser#'d.
Since 12/68,,All firearms mfg in the USA and those imported are required to be ser#'d.
There are ATF standards for placement of the number, size of the characters, depth of imprint, method of imprint and letter/number make up.
Prior to GCA 68, handguns and centerfire rifles were required to be ser#'d in domestic production since at least '38.
It has been a Fed felony to 'remove, alter or obliterate' the manufacturers applied serial number on a firearm since the Fed Firearms act of '38.
That law stopped commercial ventures such as Griffen & Howe from 'scrubbing' the ser# from '03 and other rifles in the sporterizing process.