In 1796 optical telegraph
stations were built at Grisslehamm and Eckerö and Signillskär. Abraham
Niclas Edelcrantz designed the stations which used 10 shutters which
could display over a 1000 different patterns. Although the system did
not last long in the Swedish empire the same design was used elsewhere
The stamp shows the tower with the shutters at
Signillskär, with a map of the Bay of Bothnia. A line shows the line
of sight between the stations.