Single knob shower faucet leak. Please helpleak, shower, batch

[SHAWN hansen]

So I have a single knob shower faucet that is leaking bad. I dont see any brand name on it or any info at all. I want to fix it but have no clue on what part i need for the core. Also i removed the center screw from the handle and it doesn't come off. Could it just be stuck or is there something im missing. Any help or step by step would be a great help

 

[hj]

how about a photo from the side. That looks like a Moen stop tube behind the handle but need a better photo to be sure.

 

[Sponsor]

 

[Terry]

Better yet, remove the knob and take a picture. So far it looks like it might be Moen
1200 or 1225 cartridge