Except as provided in paragraph (b) of this section, no suit or proceeding for the recovery of any internal revenue tax, penalty, or other sum may be brought after the expiration of 2 years from the date of mailing by registered mail prior to September 3, 1958, or by either registered or certified mail on or after September 3, 1958, by a district director, a director of an internal revenue service center, or an assistant regional commissioner to a taxpayer of a notice of disallowance of the part of the claim to which the suit or proceeding relates.
The filing of such a waiver prior to the expiration of 6 months from the date the claim was filed does not permit the filing of a suit for refund prior to the time specified in section 6532(a)(1) and paragraph (a) of this section.
26 C.F.R. §301.6532-1