I’m not sure when this sign dates to—the 1970s? The 1960s? Whenever New Yorkers still referred to trains by their old transit company initials rather than the letter or number.
Anyway, it sure doesn’t match the current New York City Transit sign motif.
It’s positioned high, almost out of view, just inside the 42nd Street A C E station as you enter the station from the lower level of the Port Authority.