Matt

You are correct on all fronts.
No rifles would have left the factory requiring " aftermarket" modifications in order for it to fit or function correctly. To suggest otherwise in my opinion is incorrect.
Each and every cartridge had the necessary factory modifications to the magazine box and followers to get correct function.
I dont have experience with the 6.5x55 , it may well be that this chambering sacrificed capacity and was not necessarily published.

No doubt Smiths who thought they knew better did fiddle with modifications.

My B type 9.3 is the same as yours Matt...and definatley a factory initiative.