I have an old tap that was on a 50's era keg fridge that has the extra valve, I never knew what it was for. Very interesting.
The old dude I bought it from, had to quit drinking and was selling his personal kegerator. I only hope I live as long as he had. The Fridge bi the dist just months from when I bought it, and the tap always leaked. I still have it though.
I take it that this is the ultimate faucet?