The postage and other charges (if any) on an official postal article can be properly prepaid only by a proper service stamp or stamps or by a proper impression or impressions of stamping machines. Where, however, an ordinary postage stamp instead of a service stamp is used on an official postal article, the article shall be recognized as an official postal article by the Post Office; but the irregularity shall be brought by the Officer-in-charge of the post office at which the article was so posted, to the notice of the officer concerned and, if of frequent occurrence, to that of the Postmaster General. Service stamps cannot be used for prepayment of postage on private correspondence.
