An Official Proclamation letter sent to Helsinki on January 3, 1774, during the time of King Gustaf III, concerning the delivery of grain to the Royal or Military Storage House... The wavy line (known as a meander) with two horizontal lines through it and three stylized "crowns" in the spaces (all together known as a Crown Coil or Kronoslinga) was the indicia that this was to be sent through the Royal Swedish Crown Post... The two feathers (out of three possible) indicate it is a very urgent message.
Some sources suggest that black and white feathers were also used to indicate that travel should be done by day and by night. This philatelic innovation was commemorated on a Swedish stamp in 1984.