This page describes a catastrophic (read "expensive") failure of a distributor rotor in an MG TC. I'm posting this to let other owners know about the problem I had, and to suggest a way to avoid having this happen to you.
Here are three styles of rotors I've used in my TC's:
The first, which I called "pinned", uses a drive screw to hold the brass wiper in place. The photo below shows the parts of the rotor. Now, I didn't take it apart for this photo — it's in pieces because it came apart in my distributor, with the engine running (fortunately it was idling at the time).
Note that the drive screw is simply pushed into a hole in the plastic; it is held in place by friction.
In my case, the rotor failed, presumably because the drive screw backed out (the plastic isn't cracked, which suggests the screw backed out cleanly). The drive screw and the brass wiper fell down through the distributor, winding up at the centrifugal weights. They wedged between the weights and the distributor housing, locking the distributor shaft to the distributor housing.
Once the distributor jams, one of several things can happen. The worst would be the camshaft gear breaking, as that would necessitate serious internal engine work. Or the distributor gear could shear off, which is less work to repair. In my case, neither of those happened. Rather, the entire distributor began to rotate. There is a pinch bolt that is intended to prevent it from doing this under normal circumstances, but fortunately under these unusual circumstances, it rotated. This is very tough on the high tension leads, and the primary wiring, but it also stops the engine cold because it destroys the spark timing.
Having had this failure, I will no longer use the pinned style of rotor.
According to the Moss website, the riveted style of rotor can fail because the rivet goes nearly all the way through the plastic of the rotor. The spark, over time, can create a pathway through the thin plastic and short itself to the rotor shaft, leading to a difficult to diagnose miss. That seems possible, although I've never experienced it myself.
The final picture shows the Moss Premium Rotor, which is what I'm now using. It has several advantages over the other rotors. One is that the brass wiper appears to be well integrated into the plastic of the rotor. Another is that it's a tight fit on the distributor shaft (the others wobble).