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Found on 1930s/40s irons and other heating appliances!
Ditto here, not that that's surprising.

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Could you wire it to an old GPO headphone jack style connector?
Yes, that's what I meant when I said a 1/4-inch jack. And although the majority of single phones in homes were hardwired back in the days when this set was commonly used, those which were on a portable plan would have used just such a jack.

Personally, I think the old GPO jacks were far superior to the modern modular BT plugs, but of course these days everything is done on the basis of how cheaply it can be made. [Linked Image]