Could it be a coolant pressure guage, intended for aero-engines with an evaporative cooling system?

Here’s what Aeronautics, Volume II (circa 1938) has to say about them:

“This is usually only fitted where the engine-cooling system is what is known as “evaporative”. In such systems the water is allowed to boil and the steam is subsequently condensed and returned to the engine in liquid form. A positive pressure is created in the system in order to keep the vaporising temperature somewhat above the normal boiling point and so reduce the quantity of steam generated. The guage indicates this pressure.”

It wouldn’t be unusual for the Air Ministry to stock up on instruments for monitoring equipment still not available to the service. So, gauges in stock awaiting engines never properly developed.

The only other suggestion I can offer would be water injection, but I ain’t no expert on aero-engine technology?