Commission numbers are irrelevant today...they were internal numbers used while a car was being built before the VIN was attached....MG just didn't remove them.
But, if you want to read them:
G = MG
23 = MGB; 52 = MGC; 75 = MGB GT V8
N = roadster; D = GT
F = Bodies Branch, Coventry; P = Pressed Steel, Swindon; Z = Pressed Steel, Cowley
101 to whatever = body number (but the body number doesn't equate to anything outside of Abingdon's internal records.
Plus, some cars had 2 commission numbers based on where the body came from & what it was going to be after factory assembly.
Clausinger has 3 or 4 pages on commission numbers & its just as confusing as what I just said (maybe moreso!)