Not gonna be much help here but I"ll tell you where it is NOT listed at 
It is not listed in the Product Service Letters book availiable for T Birds.
In the T Bird book offerings it lists P-307 (dated June 5th 1956) skips P-308 and lists P-309 (dated June 8th 1956).
So whatever it is it applys specifically to passenger cars and not to T Birds.
I also wish that ALL of the service bulletins were availiable.
Good Luck in your search.
Oldmics