The provisions of paragraph (c)(2) of this section shall in no event be construed to extend the period described in (c)(1) of this section for making such election. Notwithstanding the time periods prescribed in paragraph (c) (1) and (2) of this section, if no election has been made before a suit or proceeding described in section 7422(a) is maintained or a petition is filed in the Tax Court for a redetermination of a deficiency for any taxable year within the presumption period to which the election would apply, no election may be made except with the consent of the Commissioner which will not be given unless no appreciable delay in the suit or proceeding will be caused.
An election by an electing small business corporation shall be invalidated for all years in the presumption period to which it applies if a person who was not a shareholder on the date of election becomes a shareholder during the first three (or five) years of the presumption period to which the election applies and does not, within 90 days after the date on which he becomes a shareholder (or, if later, 90 days after March 14, 1974), file a consent required by paragraph (d)(2) of this section. Invalidation of the election by operation of this paragraph will in no case affect the validity of the consents filed with such election.
26 C.F.R. §12.9