Overheating blowout, bad pump

volaredon said

Probably left over from when the head was cast, I got some wire like that out of a couple of heads that I have worked on.
Dutra talks about this in his book on these engines.

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That is correct. The foundry process for making the cores that form the open shapes inside the cylinder head and block used wires to give the cores enough strength to be handled and set in the mold. That core process was typically called ‘oil sand’. 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.
By the way of the perhaps billions of cylinder heads produced with the oil sand process, each having 2 to 4 or more wires in them. Yours is the first one that I ever heard about where a core wire moved and got into a thermostat.