I happen to have just bought a 1963 catalog including Simpson's line. This object is listed as both a microammeter and as a galvanometer, part 1327C or 1329C depending on size.
A basic galvanometer is a compass with some turns of wire around it. It can read very small current, in either direction. A basic galvanometer is uncalibrated. It is excellent for showing small current, also to indicate zero current (exact equality of voltage potential). They were often built big enough for a whole classroom.
What do you do with it?
Connect to gitar pickup, wave a magnet slowly. The galvo needle will waggle. This shows how electromagnetic systems work.
Add a series resistance, it reads voltage. There are few uses for a zero-center voltmeter in gitar amp work; you could bridge it across push-pull cathode resistors to adjust bias-balance.
The meter sold for $18 back in 1963. I bet few survive. There are not many uses, but if someone today needed one, it ought to be worth $20.