French patents prevented Australians from making double hose scuba and when La Spirotechnique licensed Siebe Gorman in England to make double hose scuba for the Commonwealth, the Aussies had to come up with something. Siebe Gorman just couldn't meet the demand and Australia was far away. Siebe Gorman could make enough because of military sales, and the French were not allowed to sell to Australia becuase of the licensing agreement.
Porpoise became the first single hose, but it was too costly for this little maker to patent it, so other single hose regulators popped up. The Ted Eldred's Porpoise came out in 1952. It was followed by another Melbourne maker Jim Ager, who made the Sea Bee in 1954.
Meanwhile, up in Sydney in 1956 Scubamatic became the next in line. The fourth single hose scuba was made by Sea Hornet, but by that time single hose had spread overseas. However, that still meant Australia was first, second and third with single hose scuba.
These pictures are of #3, the Scubmatic regulator. There was no pressure gauge and no reserve. The diver carried inverted twin tanks and managed his air by decanting.
This a rare working Scubmatic. The LP hose is ribbed oxygen welding equipment hose. There is a small mushroom exhaust valve in the rear of the demand valve.