The use of flags and pennants as communications has intrigued me. According to the Sea Flags web site, Athenian (Greek) ships were first known to signal each other with brightly colored flags. Time passed and Navies of various European countries began to devise more sophisticated communication using semaphores and flaghoists after 1750.
The current situation is depicted in the International Code of Signals and in particular, pennants representing the digits one through nine are shown in the cartoon.
It’s an easy leap to create a sudoku puzzle using these pennants as stand-ins for the normal digits. You may do this puzzle calmly and in a room that has been becalmed as well.