'75 Z Vacuum Switching Valve

My '75 has a Vacuum Switching Valve, or Transmission Controlled Advance switch. This is located on the drivers side of the engine just above the distributor. There 2 vacuum hoses for this valve. Mine has only one attached and I need to know where the other one is supposed to attach. The front one attatches to the distributor vacuum advance. Does anyone know where the other one goes?

thanks...