The Siebe Gorman Merlin Mk VI with contents gauge. It has an up stream tilt valve and an over pressure by-pass in case of first stage leaks. It has a Venturi jet which results in great breathing. The base can is cast brass and all parts are model metals, so there is very little corrosion. The cover can is made of "high impact rubber." The red cord is a reserve mechanism. The Siebe had an accessory port and you have the reserve installed. The same port could have a hookah connection, or a by-pass for flushing flooded hoses. The buyer could order what he wanted. The contents gauge was made by Submarine Products, the company that made the Sea Lion, Atlantis and Black Prince regulators. This one reads "Sous Marine", which is French submarine products, and it was obviously intended for export across the channel. Siebe Gorman owned Submarine Products., having bought the company in about 1960.