Looks like the car didn't get the factory re-call on the 7 1/4 rear. That happened in mid 66 when Ma Mopar was getting a rash of warrenty rear end repairs on Hipo 273 4 speed cars with the 7 1/4. They found it was cheaper to replace it with the 8.75. Hipo cars didn't come standard with the 8.75 until mid 66. Before that it was an option that had to be ordered. Same with disk brakes. Just a bit of early A body trivia.