Plumbing – Need help identifying a shower/tub valve

After shutting off the water, try adjusting the knob to the open position to see if there's a hidden screw. Otherwise, there may be a cap that just pops off with the right pressure and often with the careful use of a blade (I usually try with a putty knife first before moving down to sharper tools).

Remove the seats, put pipe joint compound or Teflon tape on the threads, and reinstall. If it still leaks check threads inside the rough-in-valve (housing in wall) that the seat screws into. If those threads are bad you have 2 options: You can use 5-minute epoxy on the threads or you can replace the faucet.