Same thing happened to my TB IV distributor. Had about 400hrs. on it. The front of the bushing was worn badly. It looked like an oval and actually ruined the shaft also. I talked to Bruce and his guess was that the slope on the cam lobes wore down and the cam was walking forward.

It's possible. The only thing different in the engines I took down was the high-volume oil pumps that were installed at the last refit (along with Bulldog cams), but I find it hard to believe the pumps caused this. But these engines didn't show any evidence of cam movement either. Go figure.