.22 Magnum will not work, it is both too long and the casing is larger in diameter.
It would be unusual for a rifle of 50’s vintage not to be chambered for standard velocity .22 long rifle as well as .22 short and .22 long. I’d wager it’s an earlier gun that your Dad may have purchased in the 50’s based on the pictures I have found on the web. As far as shooting it, it will probably be safe, I have a number of .22’s from the 20’s and 30’s that work just fine.
Shorts are still available (CCI would be the best bet), but avoid the longs, they are no better than the shorts and less accurate - a real turn of the century orphan cartridge.
“Shorts are still available (CCI would be the best bet), but avoid the longs, they are no better than the shorts and less accurate - a real turn of the century orphan cartridge.”
With all due respect, that is just nonsense. I take it that you are not a shooter, are you?