Since the grip frame and grips just fasten on with screws, and since those who know say it never left the factory that way, then its obvious someone changed them after it left the factory.
Since you dad wasn't into that kind of thing, then he bought it that way. Fine. IF your dad thought he was buying a factory original then he got taken. But if he just bought it because he liked the way it looked, then thats fine, too.
The only person it would matter to is a Ruger collector, or someone trying to get collector price for an "as issued" original, which apparently your gun is not.
Its a nice gun, its a Ruger, and its from your DAD. Enjoy it, and don't fret that IF sold, it would not be worth as much as a "collector piece". Just because it won't bring a collector's premium price is no reason to disparage the gun, or the OP for asking about it.
Its quite possible that the gun was bought "new", meaning unfired, not "new" meaning totally unaltered. Back in those earlier, more trusting days, any gun that had not been fired could be called "new", no matter how many owners it had been through. If one of them had changed the grips and grip frame, it still would be considered new, especially since it is still all Ruger parts. "Yep, new, never fired, its just purtied it up a bit..."
Quite possible, I think.