Yes, Dave.  It does have AC.

so I guess the only function of the duct is to provide vent air back to the passenger side vent opening (the one that is manually operated by the pull handle).

Yes, that's pretty much it, Dave.

That part of the HVAC system was evolving and changing a lot through those years. I think factory installed A/C started in 74 with the 260, and in that year they had the fresh air floor vent pull knobs on both sides. They pulled the air in at the front of the fender frame channels through those plastic ducts on both sides.

Sounds like 75 and early 76 had the accordion vent and fresh air floor vent on just the passenger side. They still pulled air in through the right side frame channel, but only the right side.

For 77, they had redesigned much of the sheet metal, including the frame channels and the cowl area where the windshield wiper motor lives. So for 77, they went back to fresh air pull knobbies on both sides, but they pulled the air from inside the cowl area instead of through the frame channels. Just another reason why the 77 is the best year.