This topic caught my eye simply because it's for Fastbacks (I like Fastbacks. I have several 65-66.).
I will check the 67 & 68 assembly manuals and see if the same part numbers are listed for both years.
The 67 manual indicates a C5 part number, the "teardrop" design. (I like the "special" description.) The 68 manual indicates a C7 part number and the drawing indicates a round design. This would imply that the change was made in 1967. Looking at the C7 design, I can understand why, the "teardrop" design is slightly more difficult to "assembly line" adjust.
It some what surprises me that Ford did not just use a hood bumper on the fold down, other than the open top on the rubber they are just the same.
They look the same but may be a different diameter. ("Hindsight designing" is fun.)
Jim