Rob;
I know that under the old system of stamping gauge a gun was considered "In Proof" until it took the next larger gage. Thus a gun stamped 16 (.662") was "Out of Proof" if it took the 16/1 (.669") plug. Under this system if a new gun was .668" it would be stamped 16 & taking a mere .002" out would put it out of proof.
Basically it was my understanding the reason for the change was to show actual dia at proof & allow .008" (.2mm) of mat'l to be removed without taking it out of proof.
I am however unable to confirm this to be actual fact.
