Not trying to derail the thread, possibly a good topic for another thread, (read as: move my comment as needed) Topic is on 68 Mustang ant-rattle board details, though Scott brought up 67 Cougars in the discussion too and as with MOST details and running changes I was wondering if a general rule of thumb went along with this topic for both the 67 AND 68 model years, and assembly plant specifics.
For example: (can edit as needed)
a.) 67 Mustangs NEVER used these but 67 Cougars did.
b.) 67 Mustangs used these but only found AFTER such and such date
c.) ALL 68's had these.
d.) This adhesive found used here (plant location) and then (time frame), but this adhesive found "there" (plant location)and "when"(time frame).
Maybe a good topic for further research.
I cannot recall ever seeing one in my 67 (11/66~SJ), but such things may have been viewed as "debris" and tossed out anytime along the way.