The originator of the slogan, Milton Glaser, designed the font. Whether he did the whole alphabet, I don't know. It may be that he looked at his work as an illustrator would, and concentrated on just the three letters.
I note a greater similarity with 'Courier New' in MS Word than in the fonts mentioned above. (Try 72 pt.!) There are differences, like the lower right corner of the 'N' is rounded in the logo, while it appears to be squared off in the 'Courier' fonts I've found.
The fork in the 'y' is more spread in the logo, too.
You could ask his studio, I guess,
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