Ask and I appear....
In the very first batch of mags, the front and back of the regulator were made on screw machines. The air flow hole from the on/off on the front half was always at the top where it is now. The back half of the reg had a hole drilled randomly 360 degrees in a circle that was machined into the reg body so the air would flow around and get to the hole.
John Sosta figured out that if you happened to get a reg where the two holes were 180 degrees apart, the air flow would be less than ones where the holes lined up. This was one of several things that could have caused shoot down in the first batch. John drilled several holes in the regulator so at least one of them would be close to the hole in the front reg. It worked great as a retrofit and when he did them he called them "Pro Mags".
In the next batch and all subsequent batches, we made sure that the holes lined up across from each other and the problem was permanently fixed. The name continued to float around for years and like in this case, people come across them and think they are something special. They were basically a 2.0 bug fix and it wasn't a very serious bug.
A little more AGD history for the archives.
AGD