I don't see how to remove this stem valve. It fits into this brass pipe which is not a collar--it is entirely round, and I tackled it with channel lock pliers and quite a bit of force, and it did not budge. My next thought is the stem valve simply pulls out, which is tricky. One hardware store salesman suggested vice grips & a hammer. The other suggested extracting the rubber washer with a dental pick and he thought there might be a collar underneath that I could use a shower stem socket on.
As a new user I can't insert images, so here are the links to the pictures.