Once Upon A Time In America is based on the book The Hoods by Harry Grey. Harry Grey, real name Herschel Goldberg, was a Jewish gangster living on Delancey Street in Manhattan's Lower East Side and The Hoods is a partial autobiography.  Delancey Street is in Manhattan to the west of Williamsburg bridge but at the time of filming in the early 1980's, it was not a suitable filming location. Leone opted for the area at the other end of the bridge over the East river.  Some of the houses in the area were owned by the city or boarded up or abandoned and Gabila's Knish Factory took up a substantial part of South 8th Street.

It was an important location for many of the scenes, including Noodles' journey in a rental car for his reunion with Fat Moe, the gang's attempt at rolling a drunk, Max & Noodles first meet, exterior of Gelly's Bar/Fat Moe's, synagogue & cemetery, Noodles', Peggy's & Patsy's home, Max's home, Deborah walking home. Parts of South 8th Street - Fat Moe's and the alley - were duplicated at a set near Prietralata, Rome and many of the interior scenes were filmed there and at Cinecitta Studio, Rome.

South 6th Street, Williamsburg

The drunk is thrown out

watched by the gang

photo (2008) showing location of McSorleys exit and Garfunkel fruits and vegetables

The drunk walking towards the gang

photo (2008)

It looks like the windows to the left of the baths entrance have been cemented over


photo (2008) the baths entrance is no longer there

from the movie

photo (2008) the laundry building is no longer there

Young Max arriving on the scene

photo (2008)

map of the area

Bird's eye view

Pure speculation but the early screenplay had scenes set in a Turkish baths.
Possibly the "Baths" entrance was built for these scenes and the windows
to the left of the entrance covered with cement or mud.