I think the general concept is right: Mayview, PA. And yes, Mayview State Hospital did have its own post office from 1922-1964 and, according to this quote from a recent article on the now-closed Hospital:
Mayview even had a post office and put its own postmark on letters mailed from there.
I could not find an illustrated example of a postmark from that location, but did find that they used a Pitney Bowes Postage Meter naming the location Mayview, PA:
More to the matter at hand, the dates the post office was in operation would lend itself to the concept that a precancel may have been used at Mayview, PA. The real question is why it's not listed in the Precancel Catalog. Maybe it's unlisted because it's scarce? Or maybe it wasn't used very often? It's hard to imagine that it has been around this long without having been recorded before.
Maybe some precancel specialists can chime in with further information.