I would like to identify a shower valve, installed about 1982.
My guess is that it is a ceramic disc variety; no leaks in 30 years.
But the trim is starting to tarnish, and I would like the option of
replacing the trim. I would also like to get some spares for disc (or cartridge?).
Several photos:


There are no splines visible, nor the end of the stem; the trim sleeve appears
to fit over the end of the stem, and the acrylic knob fits via a oval with two flat sides.

Any assistance much appreciated. I am reluctant to remove the trim sleeve wthout having a replacement ready to replace it.