"Jim Burns" wrote:

It's not a new style bayonet type two wire probe. It's an old AC
(as in AC Delco) single wire screw in RTD (resistance type device).
[...]
Does anybody in the group have a Lycoming IO-540 parts manual handy?
[...]

I didn't find anything about it in the engine manual, but in the Aztec
parts manual, look for "thermocouple" in the instrument panel section
and "cylinder head temperature" in the wing electrical section.

- FChE