I'm not too sure that every gun is tested. I visited the Italian proof house in 2005. We watched them do a shotgun and before the gun was fired the pressure bearing parts were measured and noted. After proof firing the gun was measured again and if anything had stretched that is a fail. If everything is OK then the proof marks are stamped on the gun.
I don't know for sure how many test chambers there were - we only saw one - but there cannot have been too many. Given that all of the guns from Uberti, Pietta, Pedersoli. Chiappa, Beretta and the myriad of small gun makers in the valley all go through the one proof house, it seems unlikely that time would permit testing every individual gun. More likely a batch is sent down and guns tested randomly.
The two photographs are from part of the Uberti despatch room. The other big manufacturers will be very much the same.
Chance