That was an old old stamp marking that you will find on several early 22 semi auto rifles & handguns.
It was to tell the owner NOT to use the then commonly available blackpowder 22RF rounds still in circulation around the country.
(BP loaded 22RF ammo was 'wax' coated and usually so noted on the box.)
To a lot people at the time, BP was still the propellant of choice and semiauto firearms were something new. The two don't mix of course.
As noted the Rem24 has that marking. I think the early Colt Woodsman has it too.
The warning may have hung around on the bbl marking of some arms longer than really necessary, but as part of an expensive roll die that didn't need replacement due to breakage or wear,,why replace it.