An internet search revealed that Bristol manufactured planes in the UK, roughly between 1910 and 1960. They were called Bristol Aeroplane Company. They also designed and produced their own engines.
Just taking a stab here...but they likely had the ability to produce their own testing equipment for the planes they built, including testing equipment for their tyres.
IMO...the tags on the unit kind of tells a story..
"does not record tyre pressure when inflating"...means that the tyre pressure only recorded on the dial when you turn the lever from "inflate" to the off position.( similar to the hand gauges they have today ,where the tyre pressure reading pops out the end when you remove it from the valve.)
If the tyre pressure was too high, they would turn the lever to the "deflate" position, and again check it by turning the handle off.
"Frequently return to test"......means the unit must be checked against a master gauge periodically, to ensure accuracy.
It operates like an air meter in a sense that it does deliver air, and does record pressure....but has no balanced air features like the Eco's, that stop automatically at the preset setting...
Again this is only my opinion.....