What one considers the rarest vintage regulator depends on the basis for comparison. I have often thought about buying a CG45, if only I could find one for sale. They are few and far between, but I have seen a number of them mentioned here. One SCUBA I have not seen here is the rare Porpoise CA-2.(CA was for compressed air, and 2 for two tanks) It is the earliest commercially produced single hose SCUBA, and was made specifically for two tanks. It came with a cap for one air inlet, so it was possible to use on one tank, if needed. This was made before the idea developed for using using a manifold. The inventor knew of the CG-45 but had never seen one, so his designs were unique. The CA-2 has a reserve handle on the regulator. This one has valves stamped 1953 and the tanks were imported to be used with these Australian regulators.
The regulator was made to fit between two German IWK 50 cu ft tanks. The connections used were the bull nose fitting, like those used today on welding reduction valves. I know of two of these, mine and one in an Australian dive shop. This model is one of 12,000 Porpoise regulators made, and there are two of this model. So here's my question, or perhaps my challenge. Does anyone know of a rarer regulator?
Students in 1954 using the early Porpoises at the Melbourne City Baths, site of the first SCUBA school to use single hose regulators.