SHRDLU was created in 1968.

It meant that the linotype operator made a mistake and ran his fingers down the keys etaoin shrdlu to alert the printers to cut out that line. The linotype operator corrected the mistake in the line below. If etaoin shrdlu got into the paper, somebody missed cutting out the line. I used to see it in the Seattle P-I quite a bit.

1941:First electronic computer made

1949:First commercial, stored computer program

1956:The birth of AI, Dartmouth conference, logic theorist developed

1958:Lissssp language developed

1963:Start DoD’s advanced research projects

1968:Microwold program SHRDLU

1970: first expert system

1972:PROLOG language revealed

I'm just guessin' here, but I would say "N" There used to be a nonsense word that typographers sometimes used, "ETOAIN SHRDLU", which is the approximate order of frequency of the twelve most commonly appearing letters in the English language.