This is the Hydro Twin by Waterlung. It was an unbalanced 2 stage regulator and like other Waterlung 2 hose regs the IP could be adjusted externally without taking the regulator apart. The Hydro Twin was offered in 1963 as a replacement for the tilt valved Dual Air. The Hydro Twin has a downstream 2nd stage. It also had a high pressure and low pressure port. The Hydro Twin II succeeded it in 1964. This regulator had a balanced 1st stage. This particular regulator has been dived to 138 feet by The Scuba Museum staff at Paradise Springs in Ocala Florida and it performed very well. The label has some tar like substance on it that cannot be removed without damaging the label. The hoses were in poor condition so a US Divers set were substituted.