Does anybody here recognize this valve? If so a model name or number would be much appreciated!

The cover plate says American Standard at the bottom. It is in a building that was built circa 1964 and may be original. The only lettering I can find on the valve body itself are the words "shower" and "tub" on the respective fittings.

It is leaking out the valve stem when turned on. Since the only access to the valve is through the tub tile-work I would very much prefer to find replacement parts rather than replacing the entire valve.