I looked at the assembly manual, page E9-695-11, which has a view discussing the "attachment of clip" but no part number is called out. This leads me to believe that the clip was part of the more complex assembly. It does not mean that the clip does not have a part number, most assuredly it does. However, at the end of the day it was not handled here as a separate identifiable part on the assembly drawing. What is interesting is that the center rectangular vent does call out the clips for that part. Some days you get the elevator...
I also looked in the MPC illustration, and nothing there. At the end of the parts listings in the MPC it is not uncommon to see a list of related fasteners. Not so in this case.
Then I went to Jim's spreadsheet and filtered on "AIR COND" in the Location-Application column, and "CLIP" in the Description column. Of the 5 records I see 383116-S which has a Location-Application of "AIR COND DUCT, RH/LH-PLENUM". Data sources E9-695-7 and E0-695-7. I'm assuming that data source E9 is the Electrical assembly manual for 1969, sheet 695-7. Sure enough, that is what it is. Mind meld alert. My version of this page is very challenging to read. Upon further review, this is NOT the part - see attachment. The 383116 is for the plastic ductwork.
I tried.