Then the logic next question would be when was the insert introduced? And wonder if this version was used as a service part for earlier models/part
I didn't find any info on that, but 380503-S replaced 379538-S on 6/1/66, for 1966 Mustangs (ref MPC 68Sec097-P12). Note the difference in
thread shoulder length on page 139 of the AMK Guide to Ford Fasteners (3rd ed, see page 137 for drawing) for those two screws. Included is a picture of the Ford Service box for 380503-S101, showing the insert (two are positioned on top of the box. Note the very dull silver/gray finish and the black line of the nylon insert). There's a date of 05/05 on a yellow ID label on the box. Most of the rest is in "Ford code". And last, but not least, is the 1975 MPC page for 9873.
There are two other Ford uses for this screw in the 1975 MPC and that "may" be the key. It's not a direct Mustang service part, but is for others Fords AND can be used on Mustangs.
Jim
I fixed the difference definition between 379538-S and 380503-S which is the shoulder length, the non-threaded part under the head of the screw and not thread length. The difference is all of 0.0156 - 1/64th of an inch, which I assume will make tightening the screw easier and more sturdy.
Jim