I'm reading this thread, and realize that my newest regulator is now 20 years old, a Scubapro AIR I. It still breaths like a dream. I regularly use a USD Calypso regulator too.
Several years back, when US Divers come out with their plastic regulator second stages, I asked my LDS what the difference mechanically was between the newer and older second stage from USD. He said that the LP lever had a teflon bead on it so that it would slide across the LP diaphragm plate more easily. I asked how much the lever itself cost, and he said about $0.75, so I bought one and put it in my Calypso.
Belushi talked about the Nemrod Snark regulator. This two-hose had a problem--the hoses. I've used one, and switched the hoses with a USD hose/mouthpiece system, and it worked much better. Later, I corrected one other small engineering problem, and it's now a nice regulator.
My beginning regulators were:
1959: Healthways SCUBA regulator (two hose).
1961: Healthways Scuba Star single hose regulator.
1963: AMF Voit Viking regulator (shown in the photo).
After that, I joined the US Air Force, and used a large number of different regulators, both single hose and two hose.
The AMF Voit Viking 40 regulator was a great breather for inhalation, but had a very small exhalation valve on the diaphragm. It was not until later generations that the US Navy finally accepted them (single hose regs), because of this exhalation resistance problem. However, Hans Keller used this regulator (really the USD equivalent, then called the Calypso regulator) in his record-breaking 1000 foot dive. It had a balanced diaphragm first stage, and I think the first downstream second stage on the market.
SeaRat