By narrowing it down it limits the number of files I have to look through
There were a number of different suppliers used during 66 , an unnamed company and ROCKFORD with a different head designx as shown below
Doubt you will be challenged either way if the car is shown (believe some judges will like to see the Rockfords though) - Did find the use of both on original cars during 66 up through 2268 where I stopped (its later than your focus area)
The Rockford ones i believe were carried by Virgina Mustang - don't know if other places carry them, The other version I don't believe is available except from a donor car
Looks like they are 3/8-16 bolts approx 3/4" long will remeasure to make sure