Sorry, somehow was not subscribed to my own thread, so didn't get notified of responses.
It is 6" wide at the widest point.
One family member did suggest the tombstone possibility, but it's hard to say for sure. The wear lines on the edges look like they would require repeated rubbing to produce, which doesn't seem to fit the tombstone theory. However, it doesn't really fit the plaque theory either.
My wife wondered if it was a cover for something, where it was repeatedly lifted to reveal something beneath. Seems like there would be some sort of fastener present if that was the case though.
It's quite a puzzle.