1969 vs 1970 Leaf Spring Shackles

In 1969 the rear leaf spring shackle used fine threaded stud but in 1970 Ford engineers changed to a coarse thread (16) and the threaded portion of the stud was also longer. Other than these minor changes nothing was different. Maybe this "production enhancement" was made to make the shackle more durable, coarse threads are more tolerant of abuse and will assemble and disassemble more easily and quickly than fine threads and are less likely to cross thread. In 1971 the thickness of the shackle plate steel was increased considerably and they went back to a fine thread. After 1970 Ford serviced 69-70 shackles with the 1971 shackle (D1ZZ-5630-A) so finding any correct NOS shackles for 1969 and 1970 Mustang/Shelby is very difficult. Correct finish is bare steel.