I think it's more of a general thing - like the indicator was big enough they could see the range it was in, telling them enough I suppose. Kinda like if you have a clock with hands at home you're used to seeing from a distance, you might learn to recognize the time. Also, I suppose those guys had fairly decent eyesight as well, what with either recognizing planes from far away, or being the fate of 'natural selection' should they fail.
I'm not 100% sure that's what it is anyhow, just sort of looks like one to me. It could be something entirely different. I don't suppose it's impossible that it could be a 'sensor' for a remote unit, being electrically wired back to a gauge in the cockpit - but that's a complete fabrication; I wouldn't know whether that sort of instrument existed at that time.
I bet Sandbagger could tell us, if he were to visit.