1960's Air Mail from Finland to Switzerland

Air mail letter from Ikaalinen (Finland) to Lutry (Switzerland) dated 21.9.1966. 

The 0,50 marks definitive series stamp on this cover was initially issued on 2.1.1963 and eventually had a long print run (13 separate batches spanning about a decade) and with numerous different papers. The stamp on the cover has faint fluorescence when examined in UV light making it one of the later print runs.

The 0,50 stamp is singly used to cover the entire cost of foreign letter and air mail surcharge of the second lowest air mail weight class (10-20 grams). Foreign letter rate from Finland to Switzerland was (during the period 1.1.1963 – 31.12.1969) 0,40 marks. Air mail surcharge was (during the period 1.1.963 – 31.10.1967) 0,05 marks for letters weighing 0-10 grams and additional 0,05 marks for letters weighing 10-20 grams. Thus the 0,50 marks singly used stamp covers the letter rate and air mail surcharge precisely (0,40 + 2 x 0,05 = 0,50). Correct rate.