The gauge that you have, I have not seen. However, the idea was first applied to diving in 1955 by Jim Ager of Air Dive in Australia. Jim told me that a friend took the idea to Healthways, who rejected it. Shortly after Healthways began to sell a thinly disquised variation. Small stem gauges were produced for US Divers and placed on K vavles. Most stem gauges rely upon a spring which must be correct for the application. Over time the spring will slowly collapse and need to be replaced. If yours has not been used under pressure, it may be just fine.
Police here in Australia insisted that Air Dive continue to produce the stem gauges for sometime, because of their 'black water' searches. The divers did not have to see the gauge, but could feel it under zero visibility circumstances. Sea Bee divers loved their stem gauges because they could just touch them, to determine the contents of their air tanks and did not have to bring it up to be seen. I have a Sea Bee regulator with an early stem gauge, and a later one which I use as a back up for a transmitter.
You have a prize which is worth keeping.