I went to one plumbing supply store and they couldn't help me. They don't distribute for Price-Pfister, but they sent me down the road to a guy who does. "Okay", thinks I, "now I know it's a Price Pfister"
I walked to the counter, showed the guy my pictures. "Oh, yeah," he says, "I recognize that mixer. Hang on." He walks to the back of the store, snatches one out of the bin, and brings it back. "Easy-peasy" he says, "any fool could replace that cartridge."
So, we talked for a few minutes about the particular fool who was standing in front of him, and the pitfalls I might expect, which were few. "Is there anything else I should know about this cartridge before I snatch that other one out of the wall?"
"Turn the water off, " he says, "that's the biggest mistake people make, trying to replace the cartridge before they turn the water off. Makes a helluva mess"
"Good advise," says I. I turned and paid the lady at the register, then came home. Turned the water off at the meter. Fifteen minutes later I was done. No cussin'. I've got the temps regulated. (Who knew you could regulate the amount of hot water that comes out of a shower, except the old-fashioned way?) I'll have to tell Milady to be careful, don't scald herself the first time she gives it a go.
For the record (and it's amazing how much I use this blog to remember stuff), it's a Price-Pfister mixer valve, #P974-042. It cost me about $26.00 and I was glad to pay it.