I am remodelling a bathroom. The current tub/shower combo that previous owners installed says "Fontaine" on the front but otherwise has no model number or anything I can find. It works fine, except that the tub/shower selector (the small one on the bottom in the attached pics) is stuck on shower and can't be turned. I'm guessing there is a cartridge there that can be replaced? Any ideas how I might find the part and replace it though? Or is there something else I can do to free it up? I hate to have to replace the entire fixture over this.