Stamping the number after heat treat seems like it would have been hard on the number stamps maybe impossible. I am pretty sure it was done before, if so a spoiled receiver could have the number noted as it was destroyed and the number assigned to a new one later. Any clue as to whether numbers were reused or just skipped?
Thinking about this some I would have stamped the number after a last inspection prior to heat treatment.
There was that irregularity first noted when the receiver was in the stock, wondering if that prevented it from fitting in the fixture where that late operation was done around the trigger pivot point.