783 Park Avenue (Apt. 17B), the fictional residence of Don and Megan in Mad Men.
The building (783) does not exist, if it did, it would be there in the middle of 73rd Street (at east of Park Ave.).
Just to give an idea on what the building may have been like, there’s 775 Park Ave.
and 785 Park Ave.
But no 783 Park.
I guess the producers did their homework and made sure that the building will not be part of the Mad Men tour, and gave the Park Avenue residents a respite since no tourists will be swarming to take pictures in front of the fictional building.
(See also the annotations of Mad Men episodes by The Bowery Boys)