With that chain attached, my first guess would be a lamp switch. No, seriously, it's driving me nuts because I know I've seen one of those somewhere before. Maybe I'm getting Old-Timers Disease. Are those two black wires going into it?
Its the chain that held the stopper (rubber plugs also used in a bath tub) that went in the ram air scoops in the hood, to be used when racing various chevy's so that they were not sucked into the intake as we went by them.