Today I investigated further what could have caused the wear on the operating arm by taking the arm out of a bell housing that I had in the garage. Having done this I can see where the arm must have rubbed on the sharp edge behind the bushed pivot holes. See photo showing bronze bush.
The bronze bushes, part number 129358, fitted had obviously worn and were allowing some vertical movement of the arm but I did find and modify a washer to pack up the lower bush and so take up the wear. See photo packing out bronze bush.
The arm also has a tolerance ring, part number 129412, that fits inside the pivot hole. I have always assumed that it is there to stop the original design of parallel pivot pin, dropping out, so when I have replaced the original pivot pin with the flared top type I have not worried about any slackness between the pin and the arm. However considering the issue again today, if there is any play and this is combined with any wear in the bronze bushes this could allow the arm to move around a bit and possibly catch up. See photo showing the tolerance ring that fits inside the pivot hole.
Note I have found that part number 129410 is being used for both the original design of pivot pin which has parallel sides and the one with a flared top which will not drop out if fitted from above so you need to check which type is being supplied if you want the flared type.