In the beginning they were the same but as the primaries got better and breathed easier the 2nd had to be de-tuned so they wouldn’t freeflow. The reg on my stage bottle is a primary but its presurized and then turned off until I need it.
If you go to scubadiving.com and look at scuba lab you can see the breathing test for the regs, some of the 2nd’s don’t breath easy at depth so if you are thinking of going into tec later I wouldn’t buy say a Sherwood Shadow now.