1881 (May 25), 2pi overprinted “R.O.” in blue, diagonally bisected and tied to cover by blue circular grid cancel, neatly-struck Plovdov c.d.s. alongside, the bisect paying the 10pi foreign letter rate, large portion of piece missing from back, trivial given the cover’s exceptional rarity, the only recorded usage of a bisected RO overprint, certificate Hitzler (2022)