By the early ‘70s the oil sand process was replaced by what was called the ‘hot box’ process. The hot box process produced cores that were stronger and dimensionally more stable and then wires were not used.
My 1974 head had around 7 of those wires in it. So they were still in use until then at least. But it's hard to see how one could get jammed through the thermostat unless someone did it on purpose.
