To start, let me show what I have found behind my shower knob...


I've tried numerous different brands but I can't seem to find a handle that fits. Nor can I find a valve that is similar to the type currently in my shower. The question is...Is there anyway to replace the handle/guard without having to replace the entire valve? I haven't tried unscrewing the four screws holding the gray part in (honestly I'm a bit worried I may end up damaging it without having a back up plan) but if I did unscrew it, could it be possible that a new valve 'cartridge' would be able to be put into the existing piece? If not, does anyone know where I may be able to find a replacement for the outer faucet by identifying the brand this is?
Thank you in advance for any advice/help that anyone can provide!