In M1-6D Districts, the provisions of Section 43-20 (YARD REGULATIONS) shall apply, except that residential portions of a building shall provide a rear yard with a minimum depth of 30 feet at any level not higher than the floor level of the lowest story containing dwelling units with a window opening upon such rear yard. On any through lot that is 110 feet or more in depth from street to street, a rear yard equivalent shall be provided within 15 feet of the centerline of the through lot or through lot portion. In the case of a through lot on which a rear yard equivalent is provided, the requirements of this Section shall apply as if such rear yard equivalent were two adjoining rear yards. For shallow zoning lots, a reduction in the required rear yard or rear yard equivalent may be applied pursuant to the provisions applicable for an R10 District set forth in Sections 23-52 (Special Provisions for Shallow Interior Lots) or 23-534 (Special provisions for shallow through lots), as applicable.