The choice is not between 9.3x72R norm and 9.3x74R; but between 9.3x72R Sauer and Sohn and 9.3x74R.From the dimensions of the chamber casts, you can see the reason for the confusion. I believe the next step would be to fire a 9.3x74R in the shortest chamber, and then check to see if a 9.3(.365-.366") bullet will easily enter the neck of the fired case. If it does,then use 9.3x74R ammo, with confidence. If not, trim cases to 72mm and load as usual, with 9.3x74R dies. If it is the S&S version, it may require "short" bullets.Bullets longer than the 286gr RN would not stabilize in my rifle.
Mike