Single handled mixer for walk in shower handle attaches to a cogged mixer and sprayer has a plastic piece that hold escutchen and the head pipe treads into that so has to be cone shaped not flat.
I know I'm not giving you a lot any help would be great.
Terry

Posting a picture would help a bit more. I am assuming you have a drip issue or something, right? This will be a ball or cartridge mixing valve. If you remove the unit from the valve body most plumbing shops can tell by looking at it and get you a replacement.