This is a Phylrich thermostatic shower valve. From the looks it is a 1st generation that used a Grohe cartridge. You remove the handle loosening set screws at the base of the handle. To remove the escutcheon plate you will need to locate a very small hole on the underside. Original screw is very small for flat head screw. You should be able to expose the rough valve. It does have service stops to isolate this valve without turning the water off to the house.