IMHO I have not seen the screws to be original for any of the Mustang Plants though I'm sure some PO's reattached them with screws. I'll look for other examples from the plants and months you listed to illustrate that.
Sure there would have to be hundreds if not thousands of cars with screws if that was the norm.
Did recheck the 70 NATB Motor Vehicle Identification Manual that states 1970-68- VIN Stamped into Metal Tab (1970 Stainless Steel, 1969-68 Aluminum) Riveted to Instrument Pane: 1970-1969 on Left Hand Side, 1968 on Right Hand Side. VIN Visible Through Windshield
Believe there is possibly a reference to the attachment method in the TSB articles and in the Dealer communications. Also have a early 70's Auto thief handbook I'll check
Are you thinking that screws were used when all of the plants rivet tools (must have had a few with all the lines and at least some back ups) were broken/not available like the radiator support seal stapler at Dearborn?