What is this?

Survey says...It's a capacitor! What it does, if it still works, by my best guess, would be to smooth out voltages headed to your gauges that take an electrical signal. If a gauge is voltage based, given how 'reliable' vintage voltage regulators are, the electical signal wouldn't be as smooth as one might see in a newer vehicle w/ a solid state regulator. Basically, it keeps your gauge needles from bouncing around. Not really necessary, but might be nice if you've got a messy electrical system, where your needles flop around like dying fish.