First HPA tank was made by Air America- which was also it's name. later models were the Air America Raptor, or the Air America Apocalypse, but the first one was just the Air America HPA tank (or HPA system, or "nitrogen bottle", etc.)
It used a regulator very similar to the back half of an Automag- I don't know the exact relationship between AA and AGD, but clearly there was some kind of working arrangement.
The first ones came out around 1994. I still have a '94 dated AA 68 bottle around here somewhere, though I didn't buy it new.
I do know, however, that HPA was specifically not allowed in NPPL events for '94, including the World Cup- but when the '96 ESPN games aired, virtually every player was using HPA, a combination of AA systems and then-new early Max Flows. (Since one of the main teams was the All-Americans.)
And somewhere around '95, I bought a "The Regulator" system from Paintball Mania (or "Nitro Duck".)