The correct rear leaf spring part number is C7ZZ-5560-A.
Per the Ford MPC, the 1968 application is:
Fastback - 390-2V and 390-4V.
Convertible - 6 cylinder, 289-2V, 302-4V, 390-2V, 390-4V.
*Excludes Shelby.
4 Leaf, 650 LB load rate.
Marty is right, parts with stencils on them are typically service replacements.