The original TR4A part was made from phosphor bronze and is able to spin, resulting in the operating pins finding a new spot to rest at each operation. The TR250, 5 and 6 part whilst physically the same as the TR4A part, was made from steel and incorporated a small pin through the operating pin recess. The change of material would doubtless have taken place on cost grounds, the pin being introduced to stop aggressive wear.
The bad news is that the fact that the carrier does not spin means that the operating pins always stop in the same place, resulting in excessive wear in this one spot.

Carriers were never a problem on TR2-4A's and they were all phosphor bronze!