Identifying valve or cartidge type in 1958 home.

[thatwasshep]

So the bathtub faucet leak kept getting worse. I have learned quite a bit like how to even ask this question... I cant figure out how to replace the handle, no cartridge ive found fits. Anybody recognize this setup? Hard to see more, whoever worked on it last dumped concrete like stuff around the inner wall. (Drywall...?) Im really new to this.