What type of screw/fastener do you use to fasten the two plastic fresh air vents to the car? (has a 'foam seal' between the vent mechanism and the car. I have my vents, I just can't figure out what screws are used?
Based on your post, you don't have a set of "1969 Mustang Assembly Manuals". I strongly suggest you invest in a set for your car.
The screws you seek are shown in the Electrical Assembly Manual for the vents. They are 55908-S2. If you look in this forum's library under topic "Mustang Hardware Spreadsheets 69-70" you will find two spreadsheets, one by part number (MHW69-70PN06.XLSX), one by use (called APP - MHW69-70AP06.XLSX) (the 06 is the revision).
This is what is shown (abbreviated):
55908-S2 / 8-18X3/4HH,REC,FLNG 11/32,1/4HEX,TPG / SCREW,HEX,REC,FLNG,TPG / 10 / DUCT ASSY-BODY@FR QTR / ELE / 69,70 / E9-215-1,E0-225-1
It is available from AMK -
https://www.amkproducts.com/ - their stock number B-12078.
Jim