Mary Lyon Cachet honoring Buckland's famous woman educator. Drawn by Robert Strong Woodward in 1947.
Though technically correct to list this as a "cachet," because of its one-of-a-kind status, it is being included in the bookplates due to its similarity. Also, sesquicentennial relates to the one-hundred-and-fiftieth anniversary of a significant event. In this case, the birth of Mary Lyon.
"a design or inscription on an envelope to commemorate a
postal or philatelic event."
-- Merriam-Webster Dictionary