Good point TG, but I am too stuborn to give up, and I managed to get it calibrated.
With the oven and pilots off I sprayed a tiny bit of PB Blaster on the adjustment screw and let it soak in for about five minutes. Then, with the control knob still in the Pilots Off position so that it is locked in place and making a mental note of the starting point, I used a small screw driver to get the adjustment screw freed up. It does not turn easily and that is probably a good thing.
From the original starting point I backed the adjustment screw down just a bit, then fired up the oven at 350 with a thermometer in the center of the rack. 20 minutes later it was at only 300 so I had over done it.
Trying to hold the control knob stem and turn the adjustment screw at the same time is still next to impossible. With the oven off (knob locked in place) I tweaked the adjustment up just a hair and turned the oven on again. After a short time it was at 350 and stayed there.
Later I tried the thermostat at 400 and it held at that temperature as well, so the dial is probably pretty acurate in that range now.