The pen and the ink were both named "51" to mark 1939, the company's 51st year of existence, during which development was completed (U.S. design patent No. 116,097). By giving the pen a number instead of a name, Parker avoided the problem of translating a name into other languages.
The pen's resemblance to the P-51 Mustang, a fighter plane used extensively during the war, had no bearing on its name; but Parker took advantage of the coincidence by comparing the pen and the plane in its advertising.
