Greetings.

I'm trying to figure out this valve construction and purpose - it is missing, and one of previous owners just glued the hole with a chunk of household thermoinsulation.
Now I'm cutting out a lot of redneck-style fixes and missing parts in a car, but can't find a replacement - it is not avaiable to buy new, and I have no other gen 1 Blazers at nearest junkyards.

I can't even find a photo of it!
All I know is just GM part number.

Does it opens airflow in or out? Seems that there weren't any vacuum actuator on driver's side, so it was only a flap with spring?
My left leg was ice cold at winter driving after 60 mph, even if right was near boiled by heater.