"... SPGs were not yet available.."
SPGs (Submersible Pressure Gauges) were introduced to the diving world and used by Commander Le Prieur in 1926- 87 years ago. Suggest that you master French and read his 1956 book "Premier de Plongee." (First to Dive)
The American SPG appeared in 1954 produced by a long defunct company called Mar Mac, According to my Kalifornia Kalculator that is 59 years ago. An SPG has been on the American market since that date and was produced by many companies through out the years, many whom are no longer with us.
Don't you enjoy the fuzzy faced divers who loudly proclaim they were diving before the invention of the SPG?
SDM
My 1960 edition of Basic Scuba has a picture of a Sportsways Waterlung complete with SPG on page 156. I also happen to have two of those regulators, one of which included the SPG. It matches the SPG in the pic, but I have no idea of whether it was stock or not.
I also have an early Healthways Scubair with a very unusual SPG on it. Pretty sure it's early 60's and the SPG is original.