Was the anti-rattle board located on the right side of the trunk floor beneath the underlayment attached to the trunk floor only with some strip caulk or were any fasteners also used? Are there any photos that show where and how the anti-rattle board was attached?
I have also seen strip caulk used on the bottom side of the board. It sticks to the top of the gas tank and placed under where the spare tire would typically be placed when held down by . The trunk mat is placed over it once installed.
Removed my earlier post rather than modify it since I mistakenly used the term adhesive that some might interpete as being something other, in this case, the dark gray sealant looking stuff that was used and not another adhesive such as the yellow stuff used at other assembly plants
The board is difficult to make out, other than the difference between the ribbed surfaces to the drivers side and the passenger side through the underlayment that covered the tank and that area as shown in the two examples (one very early one very late production)
(https://www.concoursmustang.com/forum/gallery/17/6-150322155646-17078857.jpeg)
(https://www.concoursmustang.com/forum/gallery/17/6-150322155646-17078389.jpeg)
Since I didn't find any Shelbys in my pictures with the board in place here are some examples from other Mustangs produced at Dearborn that year
Board placement with underlayment peeled back and broken on a fastback
(https://www.concoursmustang.com/forum/gallery/17/6-150322155641-17077924.jpeg)
Back side of a typical board from Dearborn with what is left of the dark gray adhesive
Hope this helps