You're making assumptions that are no longer true. Notice now that the whole building (the Mietz Building) is referred to as 128 Plaza, while in the past it was known by the addresses 128-138 Mott Street (the higher range of those numbers were to the northern edge of the block - Grand Street).

Back in the day the block looked a lot different (old tenement) than the cosmetic changes seen today. I saw the storefront a year or two (or three) after the movie was filmed, and I stand by my photo as the movie's location for the don's office.

BTW - Here's how the upper floors look in daylight (of the ENTIRE building):



Its next door to the building you've labelled 122 Mott St.