The 20 reading is about right. Evidently the sender has failed such that there is continuity to the heater coil, but heating the bimetallic strip is not opening the points–they remain closed. The 100 PSI sender seems to have its points open–if they’re open even with no voltage applied, they can never close. The ~.0.25 - 0.5 megohm ohmmeter reading suggests some sort of contamination, or you are holding the probes with your fingers (that’s about the resistance between a pair of human hands.
Here’s a thread I just came across that shows a diagram of the sender. I had no recollection of it of course…came up googling “smith oil pressure sender internals” or similar: