Hi Gary,
I have seen this happen before, it happens when either the springs in the distributor break or lose sufficient tension that the advance weights hit the side if the distributor body and continue to rub on the side of the housing until such time as they cut through the distributor, like yours has done. I have only seen it happen once to a Bosch distributor (red motor in a taxi) but have seen it happen quite a few times to Ducillier distributors on Renaults.
Is there any sign of corrosion in the distributor. In all cases that I have seen the has been signs of corrosion inside the distributor, prompting me to think that at some stage water has gotten into the distributor and started to rust the advance springs, which weakened by rust pitting, have broken.
The other thing is if the springs were the original ones they could have broken or lost their tension and came off from fatigue or crystallisation.
I hope that this helps, but what would help more is another distributor.
Keith
F4+