Thanks for your reply! This is a 1920 with very old valve stems with washers. I've changed out most of the stems, but with dread, as I like the original tilework and fixtures. Luckily, the old builder did position a closet behind the showers with a partial access panel, so good on them. If it were 50 years ago, I'd honestly try to send the original builder a bottle for that.
I am strongly considering redoing the shower valves with a delta thermostatic when I get some time. The shower looks to me like a retrofit, and the original was likely a tub only. The tilework comes only up to 50 odd inches or so. The area above is painted beadboard and has apparently held up for at least 30 years.
Anyway I'll try pulling that stem, and see if I can't sort this out! Will post back with the result. Thanks.
DIY