I want to bring this up again.
The end caps/quarter panel extensions as we understand the process were loosely installed (and spaced out from the body slightly) when the car was painted. For this reason the exposed studs and threads were painted when the interior of the trunk was painted. Often its not allot of paint depending on how much paint was applied to the surrounding panels and the upper corner of the trunk. This practice is fairly common all years and plants though the years for Mustangs there are a few places and time periods where it appears this was not done but those IMHO represent very very short time windows at those plants or a mistake in the normal practices
Between the Extension and body were a flat washer and a mastic (used as a sealer to the body) washer for each stud (6 total), and a Sems type nut on the inside (also 6). (There should be overspray on them.)
What was used to keep the extension away from the body? How many? (There should be marks of some type.)
On later types (I think 65 and 66), there was a rubber seal to the body used. When did this seal get installed? (It did not get painted.)
Was the extension removed and reinstalled with the required hardware? (That's not very efficient.)
Per the assembly manual (and observations), there was a "wad" of sealer installed over the tooling holes from inside the trunk, supposedly after paint and after the extension was installed. (It should not be painted.)