My suspicion for the gunsmith inspection in the 1990s would be that the original owner of the gun, your great grandfather, either passed or was no longer able to remember or convey the caliber information. Whoever took ownership of the rifle prudently took it to a gunsmith to determine caliber and weapon safety. Otherwise I agree with you why get it cast 30 years after he took ownership of it assuming he knew and trusted what it was originally.
Agree with the others this appears to be a nice post war sporter conversion of a wartime Mauser.