4 Mark on 2 Fr. \"airmail 1921\", overprint error I \"numeral of value 4 thickened\", in the corner margin block of four top left with three normal stamps on over franked registered letter from \"Bismarck 13.7.21\" to a mister in Heydekrug to fractional. Bismarck (well, local letter), reverse arrival stamp, envelope outside of the franking folded, expertized engineer Becker.