That's too funny.
My son James and I went to a home yesterday with a Moen Roman Tub valve. Our 1248 cartridge didn't fit either.
We reinstalled the old part for now. They are a bear to pull out too.
Remove the threaded collar and then lift out the cartridge with a wood screw.
Except that new cartridge didn't fall back down into the bottom of the valve.
Someone needs to re-invent the Roman Tub Valve. Make it something we can service.