Climate control panel vacum port question

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Hi, I'm asking for some advice if anyone can help. I am running all new vacum lines through my 71' Chevy Super Cheyenne with fac. a/c, and have ran into something I can't figure out. All documentation, specs, and diagrams I have found show that the climate control panel has 2 vacum ports on the back, and mine has 3. They are also stamped on the lever 1, 2, 3. Is this normal? Everything is stock on the truck as well. I know what vacum lines run into the 2, but am perplexed with the third. It also looks like the previous owner had it blocked of with a bit of hose with a screw in the end. Can anyone tell me what 3 vacum lines are supposed to run into the back? Thanks to anyone that can help me out, or direct me to who can. John