American Standard stem replacement problem

I have a 1950's home with an older tub/shower combo faucet - i.e. 3 dials for hot/diverter/cold. I brought the stem for the cold to a plumbing supply house and got what looks to be an exact replacement item. However, it's an odd one, and just won't turn off completely. Instead of a typical seat that you can take out with an allen wrench, this one doesn't screw in, and actually came screwed on to the large threads at the washer end of the stem. So, there doesn't seem to be anything that will hold it in place, and thus it just keeps spinning when you turn it to the right, to turn it off. It stops just fine in the fully opened position when you turn it to the left, so I'm completely stumped on this one. Anyone have any ideas to provide some help? Thanks for any assistance you can provide.
Bob Spencer Wayland, MA
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