Next phase in my painful journey... I was going to go get the cartridge before doing water shutoff. I've got approximately 40 different allen wrenches acquired over the years. Guess which size I *don't* have? The one that fits a Moen handle. Rat#$#@!!!$&*$!! So I think I have to go to Ace to buy a wrench so that I can then RETURN to Ace to find the right cartridge.
In case this photo tells one of you Pros what cartridge I'm looking for- please let me know.