Email
Dear Mr (sorry; I'm still not sure of your name),
I have checked with our engineers. Instruments bearing the AM cypher like this were generally generic; deliberately designed to be used on a variety of aircraft as in-service replacements. The black-on-white face plus the AM cypher suggests sometime after 1918 but well before WW2, by which time most aircraft instruments were white-on-black so that they would show up better with cockpit lighting. We have no means of interpreting the codes/numbers on the back, so I am afraid this is as much as we can tell you.
With best wishes,
Reginald Byron
Tangmere Military Aviation Museum.