Last night the tub drip became a steady stream. Not my favorite thing to diddle with, but I know the drill. Broke out the 1/2" drive 11/16 deep socket, a phillips screwdriver, the washers, and the valve seat facing tool. Turned off the water downstairs, came back up, then there was a slow decent into futility...
The seat took a heckuva lot of moxie to back out. Saw signs of thread damage, widened thread and partial thin crescent of brass... Not good. Backed out the stem. Looked good, except the screw at the end had lost it's head, probably corrosion. The washer remnant was a cone holding onto the stub of the retaining screw. The remnants of the screw are somewhat corroded and will not turn.
Picked up a 3/8 NPT plug at Ace. Turns out to be wasted effort, there is an internal passage and without the stem and washer, the passage is unsealed...
Thankfully, the fixture is easy to get at. Since the hot water tap is froze up, I probably will pull the old fixture, cut the water feed (1/2" copper), put in valves and flexible risers, and put in a new fixture. This one was installed before I was born. Maybe my parents as well...
Further action: Got the stem drilled. Tapped for 8-32. Short. Grr. That's why you don't move the machining fixture until everything works. Chucked the stem up again, drilled down about 1/4" more. Now the screw fully reaches the washer and has some compression. Now to reassemble things, add some silicone grease on the stem threads.