The card below caught my attention first because of its date. It was sent from Petrograd to Switzerland in May 1921 and arrived the same month - the receiver mark is on the picture side. So it was sent within the scarce 1920 - 21 period when foreign mail services had just re-started. They were all suspended in January 1919 and resumed in June 1920. It's a registered postcard and as seems to be normally the case, it is franked at the registered letter rate of 10 roubles using a 10 kop stamp revlaued according to the March 1920 x 100 revaluations.
But then I looked at the Esperanto Star and Flag printed in pale ink on the card and realised that this is in fact an old Imperial formular card which has been recycled with these symbols on the address side and Zamenhof's picture on the correspondence side. All Imperial postal stationeries with a face value were invalidated on 1 January 1919 but continued in use as blanks, as did formulars without a face value.There were huge quantities available and the practice of overprinting them was common in the early Soviet period. The dealer Tarasoff did it in Archangel and the Soviet Philatelic Association did it in Moscow. You could probably make an interesting collection of all the different overprints, say from 1917 through to 1925 or so.