Isomac Venus Boiler

I've got an Isomac Venus that I'm about to PID

I purchased a washer thermocouple and was going to use that, but after
taking everything apart, I found a small threaded well on the rear of
the boiler , just above the temp reset

Has anyone out there taken apart their Venus ? If so:
Do you have that well? It seems odd there , completely unused
Anyone know what the threading/size is on that? I can't get to it
completely
Anyone know if its just a well, and i could slip a theremocouple in
there, or if there's something else to it?

Thanks

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