Ruger 450:

Checked the external areas you mentioned and no markings seen. The barrel has no caliber designation on it, nor other markings although I'm sure it's not a Krag military barrel, because those had holes drilled on top for the rear buckhorn-style sight. None on this one, nor any indication of filled holes.

I've read that some custom Krag rifles, as well as the sporterized Krag carbines that were turned out at one of the U.S. armories for the NRA in the 1920s & 30s, were re-barreled with 1903 Springfield barrels. Don't know that for a fact, but it was mentioned in one of the Krag reference manuals.

Maybe Gramp's rifle is one of those, or has a Winchester .30 Gov't barrel but, again, no marks either way.

I'll check the other areas when I have time. Thanks!