1972 dart - stopped running while at 50 mph ? no warning

Punch through can be from several things including say, a crack , void or defect in the rotor. This primarily happens with high energy ignition systems. Another cause is ANYTHING which causes a high voltage condition in the secondary circuit, meaning the path from coil--wire--cap---rotor--to the plus towers---plug wires---to plugs--to cylinders

Bad wire, broken plug, disconnected wire, burned valve or other lean/ misfire condition. This causes an overvoltage condition, and the spark voltage skyrockets--and just like lightning---looks for the easiest path to ground. If the rotor is somehow compromised, material quality, humidity, dirt, etc, it can arc across/ carbon track or punch through