The optional provisions of Section 23-72, inclusive, may be applied to zoning lots containing buildings used exclusively as single-, two- or three-family residences in R5 and R6 Districts without a letter suffix in the portion of Community District 12, in the Borough of Brooklyn, that is bounded by 39th Street, Dahill Road, Ditmas Avenue, McDonald Avenue, Bay Parkway, 61st Street and Fort Hamilton Parkway.