The Swedish poet and inventor Abraham Niclas Edelcrantz developed a telegraph system based on 10 keys in 1794. His book “Afhandling om telegrapher och försök til en ny inrättning däraf” can be found in chapter 4 of Gerard Holzmann “The early history of Data Networks“.
Three flaps each represented a 3-bit binary number. With the 10th flap, each code was preceded by an A. Altogether, this resulted in 1,024 code words, which were decoded to numbers, letters, syllables or phrases by means of a codebook.
Depending on the flap size, distances of up to 32 km could be bridged.
Due to its design, the system could also be used at night. For this purpose, lamps were placed behind the flaps. The codes used at night were complementary to the daytime codes.