Simmons Temptrol Shower Valve Leakage

Have the pretty typical Simmons Temptrol (single large knob) hot/cold water adjusting control in my shower.
Have a leak in the plaster in the general vicinity of underneath the shower, but not exatly.
Realize this is a tough one to answer, but what is the "history" of, and experience with, these units actually leaking to the outside ?
By this I don't mean allowing leakage thru the shower head, but actually leaking to perhaps the plaster in the room underneath.
Was wondering if there is a history of this, or they are pretty reliable in this regard ?
Hate to start digging it out if it isn't the actual culprit, or likely to be the problem.
Thanks, Bob

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