Sam,

The only thing pre-64 on the gun is the bolt and receiver. It is a 416 Remington. You are correct a Rigby will not fit in a 375 length action.

I have used both 416 Remington and Rigby on large game and anything doing 2300 fps has plenty of penetration. What I like about the 416 Remington is that it is much faster as it is a shorter action. The pre 64 action is my favorite and the most classic looking.

Some of the early 416's like the Taylor I believe are based on 458 brass and operate at a very high pressure, the longer 375 case or cylindrical brass make for a better cartridge.

__________________
Len