Old Moentrol - Which Cartridge?

We built a house in 1976 and put in a shower with a Moen single knob hot/cold control valve. The escutcheon has Moentrol stamped on it.

Taking off the escutcheon plate the valve appears to be fully symmetrical left to right with a large slotted face (balancing spool?) located right above the knob axis on the vertical centerline.

The unit works OK but it's getting pretty hard to push/pull the knob. I see that later models use Model 1200 and 1225 cartridges. I haven't tried to take it apart. Assuming this thing has a cartridge, which model cartridge would it be?

Any advice appreciated.

The original cartridge is probably a 1200, all brass. But the plastic 1225 is interchangeable with the 1200.

Thanks a lot for the information. I ran across these notes for pulling the 1200 out:

"To remove an old corroded Moen brass shower cartridge when all else fails, pull the center out of the cartridge with a pair of pliers or vice grips. Run a 1/2 tap in the cartridge and remove tap. Place a 1 and 3/4 inch long wooden dowel in the cartridge and run the tap in until the cartridge frees up. This took more turns on the tap than I expected but it worked. Clean the valve body with a tooth brush and white vinegar and flush out the brass filings by turning the water on for a second before replacing the new cartridge. I neglected this last part and the shower head filled with filings."