"gripper pattern" u bolt question

When did Chrysler install flat u-bolts?

My original 62 8 3/4, 64 Sure-Grip (tapered axles and all), even my bb 4 speed 67 with torque boxes, none of them came with flat u-bolts.

Recently swapped the rear out of my 64 with a later rearend that had regular u-bolts and 4:10s, changed the springs in the process, neither housing was damaged.