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There is a natural involution on Tamari intervals, namely reversal and exchange of bounds, with fixed points counted by 1, 1, 3, 4, 15, 22, 91, 140, 612, 969, 4389, 7084, 32890, 53820, ... One expects an instance of the q=-1 phenomenon for these numbers.
Oh, I see that I had somehow missed the point. But then one can truncate the top vertex and iterates this kind of truncation, adding each time 2 vertices and one face.