We are not sure what guage was installed next to the barometer in the conning tower, but have seen this two guage mounting bracket on other boats. It was probably a thermometer. The diameter of the brass bracket is 4 1/2 inches.
The barometer installed is also probably not the correct type. It was probably the same manometer used on the dive station, usually made by Taylor, usually labelled "TYCOS". Always had "US NAVY" on dial. Early fleet boat units had a siver dial, later "Balao" class subs had a black dial, white lettering that glowed. Dial goes up to 36" of mercury.