Imperforate stamp (proof).
A postage-due stamp is a postage stamp used to collect postal charges from the recipient: when a postal item is sent with insufficient postage or with unpaid postal fees; and for postal items whose delivery must be paid specifically by the recipient. Postage-due stamps were distinctive in that they were not sold to the public; instead, postal clerks affixed them to correspondence for which the sender had paid less than the current rate, and the surcharge was collected from the addressee upon delivery. In the USSR, postage-due stamps were issued until 1925.
From May to October 1925, two series of postage-due stamps of various denominations were issued and used until February 1, 1926. Later, postage-due stamps were replaced by postage-due handstamps bearing the inscription "To Pay" or "Surcharge".