Prior to The GCA of 1968, S/N's weren't required on .22's.
Prior to the 1968 GCA, serial numbers weren't required on anything
, not just .22's. It also wasn't illegal to change or obliterate a serial number. (Since it's now illegal to buy and sell guns without serial numbers, there is a legal process in place for legally transferring a pre-68 gun with a "pre-obliterated" s/n, but this is a bit off topic. Originally non-serialized firearms are grandfathered.)
Inexpensive .22 rifles were often produced without serial numbers in the pre-GCA era because (a) these rifles were regarded as cheap tools that were easily replaced and (b) applying serial numbers costs money, so eliminating the s/n allowed the manufacturer to save a few cents and hopefully undercut the competition's prices. Interestingly, gunmakers would apply serial numbers to more expensive .22 models as a sort of premium feature. For instance, Winchester did not serialize the Model 69 bolt-action repeater, but they did
serialize the fancier and more expensive Model 75, even though it was based on the M69 action.