Heater blows, but doesn’t redirect air to defroster.

I happen to have another heater box out of another 74 A/C Dart laying around. I see a cable that operates what appears to be a door that controls whether air blows by the heater core or not. There’s a vacuum canister on the other side with 2 hoses coming off of it. Sucking and blowing on them moves a rod a little, but I am unable to determine what that actually does