I can't be sure.....but I bet the piston just changes the upper pressure rating of the reg, in other words the point at which the reg will just start to blow off air. Either that or the piston size is changed to simulate more or less spring pressure, and therefore more or less travel to increase or decrease the upper and lower pressure output of the reg.
I wouldn't worry about it.....if it's working you're good to go. I think they all come standard as low pressure, unless you specify that you want otherwise. Don't quote me on that though....
ETEK 3LT, with 14" Ti boomstick and cure 3 + bolt F/S
Vanguard creed eyes, membrane, bolt pin, detents F/s[/color]
Old school angel poppets/cup seals, spring detents(not sure which models) F/S