Section 181(2) in The Indian Post Office Rules, 1933
(2)Envelopes bearing the inscription "Official Paid" printed at the top right hand corner of postal articles in lieu of postage stamps shall also be considered as official postal articles and shall, subject to the provisions of the Note (2) to Rule 190 be eligible for transmission by post under the conditions prescribed in sub-rule (1) of this rule. These envelopes shall, for the present, be used by Military Headquarters. Formations and Units only. The use of such envelopes shall be confined exclusively to ordinary official letters of the inland post. These shall not be valid for use for registered or insured letters or for parcels or for postal articles of any other category. These envelopes also shall not be sold at any post office.
