These hood bumpers were used on the '68 Shelbys as trunk bumpers, but not on the GT/CS.
If you open your trunk, and look at the sheet metal part of the trunk inner edge, you'll notice a flat round spot, stamped into the metal (on each side).
This is where the Shelbys used these bumpers. They drilled small holes, and then screwed these all the way down to the metal (flat spot).
The GT/CS did NOT use these bumpers. Instead, they used those grey foam strips, the attached onto the underside edges of the trunk lid. The foam took "the hit" as it were, when the trunk was shut/slammed/closed, etc...
If you look at a (left or right) rear quarter piece when the trunk lid is open, you'll notice an inner curved part of the qtr. panel, that you can't see if the trunk is closed. This is where the foam piece makes contact when the trunk lid is closed.
Just because a '68 Shelby uses a certain part, one can not assume that it's necessairily used on a GT/CS.
Paul N.