Looks like I got my money's worth (or not). The probe appears to be defective. It does have continuity, about 13 ohms which is in the ballpark, and the end is not touching the wall of the wye (first two things I checked)!
But the output signal is a very slowly drifting-upwards level of 1.5 to 1.8 millivolts, even with the engine warm and revving, way too low according to K-type thermocouple charts with a 200-300F hot junction. The gauge scale starts at 300 anyway. I drove up and down the road and got into boost, but the needle still didn't budge off the low pin (300).
I also hooked up the leads to the Triplett panel meter and it also gives roughly a 150F reading. Reversing the leads makes the meter read downscale instead of upscale, so there is
some kind of thermocouple junction out there. But I'm betting it's shorted in the cable or at the probe body, instead of the two wires meeting at the probe tip like they're supposed to.
Tomorrow I'll apply a few millivolts DC directly to the gauge and see if it responds. My bet is that it probably works since it sweeps the entire scale and back at power-on, but at this point I'm not sure of anything yet