That is an elegant solution to getting the head space to repeat. When you install the barrel it is touching the original internal shoulder to set the headspace. I would guess the end of the receiver was faced or ground square so the locking nuts have near 100% contact when tightened. The end of the receiver was not faced  square when made at SA.
I think the stiffness of the assembly will be greater with nut than the original system that only bears against the internal surface.