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Here is a March 1923 commercial registered cover from Petrograd to Berlin, the stamps cancelled with an oval railway station PETROGRAD NIKOL. VOKS. 3 3 23. But the Registration label - in French, for foreign mail - is pre-First World War, reading S.PETERSBOURG gare Nicolas.
I have seen this before - but possibly it was this cover that I saw. It shows - if you like - how relaxed Soviet authorities were about using up old Imperial paper stocks.
The cover is correctly franked to 10 roubles in 1923 new currency (each stamp revalued to 1% of its former value) or 10 000 000 roubles in the immediately preceding currency period - it is probably some "collector" who has written the "10,000,000 Rubel" at the bottom of the cover back, underlining it twice in case we miss the point.
The real challenge is to find a 1924 cover which manages to have postal markings or labels showing all three possibilities: St Petersburg, Petrograd and the new Leningrad ...