It is probably a safe bet that there are already a lot more 375 Rugers in the world than there are 9.3x64's.
I think it's a safe bet that the opposite is the case.
The 375 Ruger is hardly sold outside the US and Canada. As big as that market may be - at least in Europe there are still thousands of 9,3x64 being used. Looking at an online platform for used guns for Germany only, there at least a dozen 9,3x64 bolt actions for sale at any time.
Pretty sure there are quite a few spread all over Africa as well. In Russia it is used by the military as a sniper cartridge. Cheap ammo available from there.
Just my 50 cents.
