From a period publication titled APARTMENTS OF THE BETTER CLASS
3 Flat using a interior design for what were obviously upscale residents. A good indication of this is the inclusion of a passenger elevator in a 3 storie walkup; a very rare perk even by todays standards. Then you have a driveway curb cut with access to both the main building and garage parking.
We have what has become the required servants room with its own private bath. Now open wood porches here; but a enclosed stairwell.This is not moderately sized unit.
I think the designers were anticipating adjacent future construction in that zero lot-line elements are present in the layout