Given the way they were used, the production of Helmet Test loads was probably tiny compared to other types of P08 ammunition. In addition, the distribution would be limited to those locations where the testing would be done, which was relatively few places compared to those where normal service ammunition would be located. Proof loads were traditionally used on every weapon produced, but Helmet Testing was probably done on only one helmet in a hundred or more likely, one in a thousand.
Similar testing is done on aircraft canopies and on highly pressurized aircraft items and many other things. Fighter canopies are interesting. The outside surface must be very hard and tough to withstand penetration or fracture from things like bird strikes. The inside must fracture relatively easily from pointed object impact to allow the pilot to break up the canopy and escape if the canopy ejection system fails in an emergency. In each cockpit on an F-4 was a knife that looked like a black hunting knife, but with a blade only about 2" long and very pointed. Four or five hard stabs to the inside of the canopy will shatter it and allow the pilot to climb out.
To make sure the canopies satisfy these requirements, there were machines at the canopy production facilities that used a machine to do a series of carefully caliberated attempted penetrations to the outside of the canopy, and the canopy must stay intact from all, including a full penetration without the canopy shattering. The inside of the canopy was also tested, in this case to ensure it would shatter.
Tests were done periodically during production and whenever anything about production was changed that might effect these characteristics of the canopy. This could be changes in the plastic supplier or changes in the production batch of the plastic or in the plastic processing (like melting) or molding or polishing or any thing else that might effect the critical characteristics of the canopy. A number of canopies are used up by this testing so the production line maintenance and plastic acquisition are carefully managed to minimize the number of canopies used. These were pretty standard quality control and assurance procedures.
My experience goes back some decades so computer tracking and control of production may make it simpler now. Still, I expect helmet testing was similar in Germany during WWII. A helmet would pass if the helmet test bullet did not penetrate the helmet. I wonder which troops got the helmets that were tested and passed? I could see them going to training units or ???
It is interesting that earlier German Helmet Test rounds (from before 1944) have not been found. Perhaps they were in another caliber than 9mm P08, or perhaps it was not considered necessary.
Still, the very low production requirements for this ammunition may mean that none have shown up. Given the controlled conditions of it’s use, the vast majority of it must have been fired and very few rounds leftover unless production was terminated, like at the end of the war. I know of one round by RWS from 1944 with a yellow cms and primer seal which also may be a Helmet Test round.
Proof rounds fall in a similar situation, but some in 9mm P08 show up dated back to 1939. However, I have seen a Polte box label for P08 proof rounds dated in the 1920s, bot no proof cartridge from the 1920s or early 1930s.