Product review (3 stars): Yup, its a Humidstat.
I had an electronic Aprilaire humidtstat that stopped working. I bought this to replace it cause I need SOMETHING other than me manually switching the humidifier on and off.

This unit is COMPLETELY mechanical, has no power requirement at all. It acts as a normally open relay that closes when a membrane stretches taught due to dryness, adjusted by the knob. But it does actually work, and it could hardly be less reliable than the electronic one its replacing, so its worth the money if it lasts 4 years.

I hoped being the same brand it would have the same footprint. Nope, its about 4mm narrower than the electronic one. Currently I have it duct-taped in place; now that we are past heating season I will fabricate something so that it fits the hole in the plenum properly.