Mark Lamster thought that was the Daily News Building behind Don, and I had to agree. Through the wonders of Google maps, I see 300 East 42nd Street, built in 1963 and designed by William Lescaze is a glass curtain-wall building directly across the street. I’m going to try to find an interior photo, since 300 still has opaque spandrels, which is what most of the 1960s curtain wall towers used to cover up the radiators.
That said, Second Avenue seems about right for a fallen Mad Man, so I have to eat my photo essay. This image is from the rather wonderful rental brochure from the Avery Art & Architecture Library.
I am sorry I doubted Matthew Weiner’s quest for accuracy.