These are fictional sailors that were required by law to be on the books of every ship in the (British) royal navy. Weird.
"An appellation given to a certain number of men, according to circumstances, in every hundred of which the complement of a ship shall consist, who are directed by Act of Parliament to be borne on all his Majesty's ship's books as able seamen, the produce of whose wages, and the value of whose provisions, are applied to the relief of poor widows of commissioned and warranted officers in the royal navy.
"For the due performance of this benevolent scheme, every captain, or other officer, commanding any of his Majesty's ships or vessels of war, is directed to enter on the books of the ship or vessel he commands, as part of her complement, one or two ... fictitious names in every hundred men ... which are to be borne as able seamen, under the appellation of widows' men with numbers 1, 2, 3, &c. set against them...."
—Falconer's New Universal Dictionary of the Marine, 1815
This also reminds me of the fictitious (and fictional) Lieutenant Kijé in Yuri Tynyanov's brilliant short story of the same name, who started life as a slip of a copyist's pen, received decorations and promotions, but then was exiled to Siberia. In the 1930s the story was made into a movie, for which Prokofiev composed the score, later turning it into the "Lieutenant Kijé Suite".