I checked my mounts. I have a Weaver N2 mount on a Cooey/Winchester 600. It has a vintage Weaver B6 3/4" scope on it. The scope sits centre of the barrel where it should be. I have a Weaver N3 mount not on a gun. I measured it and it will take a 3/4" scope. However, it positions the scope a bit higher than the N2 mount for guns that need more bolt clearance. The scope is still centre on the barrel.
But, I also have a vintage Weaver N3 mount still in the package (not opened) with the paper and it says for 7/8" scope.