And here is the 12k overprinted stamp in its intended use for collecting postage-due:


The Soviet Union used postage-due stamps for only a very brief 2-year period (1924-1926). As a result, these stamps are quite plentiful mint, but scarce used on cover, especially on incoming correspondence.
The card was supposed to have been sent at a #8220;printer paper#8221; / matter rate, which at that time appears to have been ½ d to the USSR. According to UPU regulations, the postage-due charge should have been twice the deficiency, i.e 1d. (I have no further information regarding exhange rate to ruble or on the other marks on this card - any opinions would be most welcome).