The optional provisions for sky exposure plane buildings set forth in Section 23-73, inclusive, may be applied to zoning lots in R6 through R10 Districts without a letter suffix, except that such alternative provisions shall not apply to:

1. zoning lots in R6-1, R6-2, R7-3 and R9-1 Districts;
2. zoning lots in R8 Districts without a letter suffix in the portion of Community District 9 in the Borough of Manhattan located north of West 125th Street; or
3. zoning lots located in the Limited Height Districts.