You have a Moen "Magnum" 1200 cartridge. A 1200 or a 1225b cartridge will fit. I recommend a 1225b cartridge as it will be easier to deal with, but there is a possibility that it may not seal. You'll definitely need a moen removal tool, otherwise the cartridge will break inside the valve. Make sure to turn the cartridge with the puller before pulling it out. More on reddit.com

Double check the old seal did not stay in the valve body. Compare the old w the new unit to verify all pieces are out. Also grease the seals on the new unit. Do NOT cut it. It will fit if it’s the right unit More on reddit.com

Buy a Moen Pull tool More on reddit.com

It looks like 2112 is a Moen single handle shower system that uses the Moen Posi-Temp valve. I replaced some Posi-Temp valves in my showers, and it was pretty easy. The newer ones are brass and plastic, and seem to be working just as well as all the old all- brass ones. https://www.lowes.com/pd/Moen-1-Handle-Brass-and-Plastic-Faucet-Tub-Shower-Cartridge/3133585 ? They usually come with a crappy plastic faucet puller, but I spent the $16 on a metal tool to get the old cartridge out and put the new one in. It helped out a lot!! The old ones can be tough to wiggle out, and this was the right tool for the job: https://www.lowes.com/pd/Danco-Cartridge-Puller-For-Moen/1085199 More on reddit.com