Did the architect first use this nickname?
30 July 1981, New York Times, "Why Buildings Grow on Us" by Paul Goldberger, pg. C15:
For a long times, I thought that nothing could be worse than the "XYZ" buildings on the Avenue of the Americas, the massive Exxon, McGraw-Hill and Celanese skyscrapers that comprise the western expansion of Rockefeller Center, so named by their planners because of their nearly identical design. The three boxy towers are banal in the extreme, with huge and generally useless plazas dulling the street life in front and straight tops flattening out the skyline up above.
10 January 2005, New York Sun, "Abroad in New York" by Francis Morrone:
Between 50th and 51st rises the Time & Life Building of 1957-9, by Harrison & Abramovitz. This is among the handful of postwar buildings to have received landmark designation. The same architects later added the three buildings, each occupying a full block front, between 47th and 50th. Wallace Harrison himself dubbed these three seemingly identical buildings the ''XYZ Buildings.''
The one between 47th and 48th was called the Celanese Building when it opened in 1973. (The avenue once had the headquarters of Celanese, J.P. Stevens, and Burlington. Might we call this the Microfiber Mile?) Next north is the McGraw-Hill Building (1967-73). Between 49th and 50th stands the former Exxon Building (1967-71).
Each of the XYZ Buildings stands back from the avenue behind a large plaza, two of them of the sunken variety that would soon be discredited by the urbanistic studies of William H. Whyte. Each building rises sheer in a scaleless mass that announces, above all, that it is large. Indeed, each comprises nearly 2 million square feet of floor area.