(ii) Where the rent charged on the base date cannot be established, the rent shall be determined by the DHCR in accordance with section 2522.6 of this Title.
(iii) Where a housing accommodation is vacant or temporarily exempt from regulation pursuant to section 2520.11 of this Title on the base date, the legal regulated rent shall be the prior legal regulated rent for the housing accommodation, the appropriate increase under section 2522.8 of this Title, and if vacated or temporarily exempt for more than one year, as further increased by successive two year guideline increases that could have otherwise been offered during the period of such vacancy or exemption and such other rental adjustments that would have been allowed under this code.
(4) Complaints filed prior to April 1, 1984 shall be determined in accordance with the RSL and code provisions in effect on March 31, 1984, except that an overcharge collected on or after April 1, 1984 may be subject to treble damages pursuant to this section.
(b) The DHCR shall determine the owner's liability between or among two or more tenants found to have been overcharged during their particular occupancy of a housing accommodation, and at its discretion, may require the owner to make diligent efforts to locate prior tenants who are not parties to the proceeding, and to make refunds to such tenants or pay the amount of such penalty as a fine.
(c)
(1) Any affected tenant shall be given notice of and an opportunity to join in any proceeding commenced by the DHCR pursuant to this section.
(2) Where a complainant pursuant to this section vacates the housing accommodation, and the DHCR continues the proceeding, the DHCR shall give any affected tenant notice of and an opportunity to join in such proceeding.