Legend has it that the Swimaster MR-12 was the first single hose regulator approved for military use. MR-12 stands for: Military Regulator, Single hose, Two stage.
The balanced diaphragm MR-12 first stage, in my opinion, was one of the finest ever made (see picture). One particularly nice feature of this simple, compact, dependable regulator is the ease with which the intermediate pressure (IP) can be adjusted. See that big hex key slot on top? Stick an allen wrench in there, turn clockwise to raise the IP, and the other way to lower it. Simple! Snap an IP gauge into the inflator hose and you are all set.
The Voit Swimaster company was sold to Mares somewhere in the early-70's and the MR-12 was from then on sold as a Mares MR-12.