Specifically, what is that little meter? I'm guessing it's a way to track milage, but I have no idea what it's called. It looks interesting though.

Not sure if any of this info would help, but it's on a 1960's Hercules 3-speed and the object in question is attached to a Sturmey Archer dynohub generator. Original posting of this bike on Fixed Gear Gallery:
http://www.fixedgeargallery.com/2004/g/lasala.htm