The barquePisagua. Her Code letters RJPT are flown on the jigger mast, above her ensign
Code letters were a method of identifying ships before the introduction of modern navigation aids. Later, with the introduction of radio, code letters were also used as radio callsigns.
In 1857, the United Kingdom sponsored the Commercial Code of Signals for the Use of All Nations at Sea, which introduced four letter flag signal codes to identify individual ships. Circa 1870, the Commercial Code of Signals became the International Code of Signals. By the 1860s, individual ships were being allocated code letters in the United States and Europe. From 1874, code letters were recorded in Lloyd's Register as part of each individual vessel's entry in the register. Generally, code letters allocated to a ship remained with that ship, although there are known cases where new code letters have been allocated following a change of port of registry or owner. Code Letters were sometimes reallocated once a ship had been struck from the register, but no two ships bore the same code letters at the same time. With the introduction of radio for communications, code letters were used also as radio callsigns.
Code letters used the twenty-six flags that represent the letters of the alphabet. The ten flags that represent the digits 0 - 9 were not used.
"The allocation and use of ship identification signal codes for merchant ships to WWII". Jeremy Lowe. Retrieved 16 January 2009.
This website has a transcript of radio communications between MS Hans Hedtoft (OXKA) and FV Johannes Krüss (DEQW).