How was this pocket ammeter used?

[Sparky]

I came across this ammeter in my father-in-law's tool collection. He got it from his father who had a Model A back in the day.

I did a quick search and found several for sale on eBay, mostly described as an "antique automotive tool." This one is missing a short electrical lead that attaches to the top terminal post.

So did this have a specific use, like checking generator output, or is it a general purpose tool?