Dave - Just sharing and not meaning to be argumentative in any way. I've never seen that style of trim screens on a regular 66 original cars early or later
Often we don't understand why others or prior owners did things and if we shared all of those sorts of findings the thread would be mighty long.
Suggest that if you feel that they are original to start collecting others that are identical from the same period of construction at Shelby and only from cars that have not been restored to start with.
Closest I have close to 6S1757 is pictures of the 6S1732 that Brant did that also had the pan heads
Wonder if the new tach kit came included mounting screws that that might explain it. Just thinking out loud
Here is another example from a pretty late example - 6S226x