Hi Sam,
I was just reading the 1962 Skin and Scuba diver hardbook(Arco 957) you sold me.
sold on may 2/2020 was on my ebay copy description.
You will not believe it on page 125 the Ice divers walking have two different models of PRO single hose regulators on.Wow never seen anyone ever using one underwater thru out the years.
Jerry