1936 air mail letter with postage due Haapsalu Vaksal AG – Strängnäs, Sweden
Attractive multifranking with five 5s Pirita stamps, sent from postal agency at Haapsalu railway station. Franked with 25s which was standard international rate at time of sending, but since air mail label was attached, postage due was charged for missing international air mail fee of 20s. Postage due amount 33c in UPU gold centimes is inscribed on letter and was calulated with the following formula: 20 (missing) x 2 x 5/6 = 33c. Same day transit postmark from Stockholm on back side.