Not just .22's, I have seen a Savage 340, magazine fed bolt gun, chambered in 30-30 that was also manufactured sans serial number.
Many of the FN mfg Browning semi auto rifles have their ser# stamped on the bbl. In Europe, many countries count the Bbl as the control part,,as in the USA the Frame is the control part (firearm).
Some of the Browning T-bolt rifles are also ser#'d on the bbl as well.
Before GCA 68, there was no regulation that said that cal .22rf long guns nor any shotguns had to have a mfg'rs ser# imprinted on them.
Some mfg'rs did number them, or some of their production,,some didn't number any of them.
Handguns regardless of caliber & center fire rifles were to be ser#'d .
GCA68 changed the regs and it then demanded that the .22rf cal long guns and all shotguns also be ser#'d as well.
So that's where the common slightly misleading statement comes from that usually states something like 'With the enactment of the GCA68 all firearms were required to be ser#'d.'
That's true,,but the rest of that unsaid is that before GCA68, all handguns and centerfire long guns were already required to be ser#'d.
There were several instances of mfg'rs not ser#ing some of their center fire rifle production in the pre-68 days.
Savage didn't ser# quite a number of the early production Model 219 break open rifles. The 22H/30-30 switch bbl rifle.
I've got one here right now for repair in 22Hornet.
That may have had something to do with them using the same action on their Model 220 shotgun,,a gun which would not have needed to be ser#'d.
Just a guess though.
The Savage Model 340 escaped w/some being unser#'d as well.That one just an oops I'd guess.
Marlin sent the first 4000+ Levermatic rifles in center fire form (256Win Mag) out the door w/o ser#'s in the mid 60's.
Marlin had orig ser#'d the earlier production .22rf Levermatic rifle,,then to save 3cents per rifle in labor,,stopped ser#'g them.
This was a perfectly legal move as it being pre-1968,,no ser# was necessary on a cal .22rf long gun.
Then a couple yrs later the centerfire Levermatic came out (64/65).
The production line made no room for the recv'rs to be ser#'d,,,likely since they had not ser#'d the other levermatics (22rf) they had been making for quite some time.
So as a result and until such time that someone finally spotted the issue,,4000+ unser#d 256WinMag Levermatics (mod62?) went out into the world.
The Treasury Dept was not happy (they were the Federal Firearms enforcement agency of the day pre-68)
But the 'fix' was to allow Marlin to issue a continuing recall on those unser#d rifles for the owners to vol return them to the factory for numbering,,at Marlins cost of course.
...Sure,,I'll send it right in...
Very few ever came back voluntarily. A few got stamped that happened to come in for repair.
Imagine the owner getting the repaired rifle back with a hand wacked thru the bluing new ser# on the left front of the frame.
We had a BATF (post 68) log for just the recalled Levermatics and their new ser#'s in the Repair Dept. It was not a long list.