Okay, this has been discussed on the Vintage Scuba website (see above link), but I'll say it again.
First, Voit manufactured the 50 Fathom, and it is not the same as the Mistral. The Voit VCR-2 50 Fathom was manufactured between 1959 and 1961, and has a downstream design for this single stage regulator. It is the only downstream push-pull lever-operated seat design for a single stage, two hose regulator ever produced (excepting the VCR-5 Blue 50 Fathom, which was the same regulator with a plastic case). This means that it gets harder breathing with reduced tank pressure (slightly, but it's there).
This is the opposite of the Mistral, which was (the old, single stage USD Mistral) an upstream push-pull lever operated regulator design. This regulator got easier to breath as the tank pressure decreased.
The one thing to know about the Voit 50 Fathom is that the venturi was not pointed correctly, and must be re-directed so that the large hole points down the intake hose. This give optimum breathing performance, but to do so the metal dowl must be removed as it "keys" the valve mechanism to a different orientation, with the venturi port pointed towards the case. I have speculated that this was intentionally done by Voit so that their inexpensive, single stage regulator would not breathe better than their two stage regulator, which at that time was the VR-2 Mariner (1956-1959). The VR-2 Mariner was the equivalent of the USD DA Aqualung, the first double hose regulator marketed in the USA, and a very hard breathing regulator.
SeaRat
