The following is information from a 2008 real estate listing for the building I believe Salinger imagined the Caulfields lived in: 3 East 71st Street. Inside, at least in 2008, it's not the humble sort of building the exterior suggested to me (despite the doorman). The building has a health club and was referred to as "one of Manhattan's most desirable white glove buildings." The asking price was nearly a million dollars.

Living Room	in 2008 Real Life	in Catcher in the Rye
	4.5 Rooms	(unknown)
	2 Bedrooms	five? (parents, D.B., Holden, Phoebe, maid)
	2 Bathrooms	(unknown)
	Coop	Apartment
	Doorman	Elevator Boy
	N and W views	W and S views
	Great closet space	closet space enough to hide in!

A gracious 2 bedroom, 2 bathroom apartment, steps away from Fifth Avenue and Central Park, in one of Manhattan's most desirable white glove buildings. The elegant living room has high ceilings, a wall of windows and a wood-burning fireplace. There is a dining area off the windowed kitchen. The oversized bedroom has a wall of built-in storage cabinets as well as a customed designed walk-in closet. The second bedroom has many built-in bookcases and the two bathrooms have been renovated. Apartment Features: North and West exposures, Partial city view Wood-burning fireplace Floors - hardwood Renovated bathroom, Marble bath Storage space Walk in closets, Great closet space Dishwasher Building features private storage, central laundry room, and health club. Asking price: $999,000.00

Obviously, the unit is not the one Salinger indicated because of the difference in the number of rooms. Further, with views out the north and west, it wouldn't include the SW window from which Holden watched Phoebe cross the street to Central Park (see explanation).
    The Dicksteins' lived across the hall from the Caulfields (see page 157-8). Holden also said there were only two apartments on their floor. That could have changed, especially given how much smaller this apartment is than his. We don't know what floor the listed unit is on, too. The Caulfields lived on the 12th or 13th floor (see page 88). We only know the listed unit is high enough to have a partial view of the city, which could mean only a tiny sliver of a view from a middle floor, but the listed unit's floor plan shows a staggered outline on the west side with windows. Only the upper three floors have a non-square outline. That means it is on the 12th, 13th, or the 14th floor. The unit could be on the correct floor and be the Dicksteins'.
    Well, that's all. This is a fiction book.

	Bedroom
Library (presumably Bedroom #2)
Living Room	a humble Kitchen