(a) Except as otherwise provided in section 6501 (c), (e), or (f), any tax for which a return is required and for which:

(1) A decedent or an estate of a decedent may be liable, other than the estate tax imposed by chapter 11 of the Code, or

(2) A corporation which is contemplating dissolution, is in the process of dissolution, or has been dissolved, may be liable, shall be assessed, or a proceeding in court without assessment for the collection of such tax shall be begun, within 18 months after the receipt of a written request for prompt assessment thereof.

(b) The executor, administrator, or other fiduciary representing the estate of the decedent, or the corporation, or the fiduciary representing the dissolved corporation, as the case may be, shall, after the return in question has been filed, file the request for prompt assessment in writing with the district director for the internal revenue district in which such return was filed. The request, in order to be effective, must be transmitted separately from any other document, must set forth the classes of tax and the taxable periods for which the prompt assessment is requested, and must clearly indicate that it is a request for prompt assessment under the provisions of section 6501(d). The effect of such a request is to limit the time in which an assessment of tax may be made, or a proceeding in court without assessment for collection of tax may be begun, to a period of 18 months from the date the request is filed with the proper district director. The request does not extend the time within which an assessment may be made, or a proceeding in court without assessment years from the date the return was filed. This special period of limitations will not apply to any return filed after a request for prompt assessment has been made unless an additional request is filed in the manner provided herein.

(c) In the case of a corporation the 18-month period shall not apply unless:

(1) The written request notifies the district director that the corporation contemplates dissolution at or before the expiration of such 18-month period; the dissolution is in good faith begun before the expiration of such 18-month period; and the dissolution so begun is completed either before or after the expiration of such 18-month period; or

(2) The written request notifies the district director that a dissolution has in good faith been begun, and the dissolution is completed either before or after the expiration of such 18-month period; or

(3) A dissolution has been completed at the time the written request is made.