RSFSR / Sovdepia: persistence of the 10 rouble Foreign Registered tariff in 1921

Understanding the franking of mail abroad in 1920 - 21 is complicated by the existence of locally-authorised Tariffs. Alexander Epstein discusses this in his article, "Lokale Tarifautonomie 1920 - 1923, Teil 1, Auslandspost 1920 - 21"  in Deutsche Zeitschrift für Russland-Philatelie, Nr. 94, 2011, pp.26 - 36

However, there is what one could call a normal or dominant 10 rouble Tariff for Registered letters, that is 5 roubles for a letter of the first weight step plus 5 roubles for Registration. This Tariff persists at least until the end of August 1921, as illustrated by the following covers. I date them according to the postmark applied to the stamps:

Moscow 4 2 21 sent to England
Tambov 18 4 21 sent to France
Syzran 30 5 21 sent to USA via Berlin
Omsk 22 7 21 sent to Germany
Petrograd  12 8 21 sent to Czechoslovakia
Petrograd  25 8 21 sent to Germany
Petrograd   30 8 21 sent to Latvia

All these covers are franked with Imperial stamps - the new Arts and Industry stamps were not released until August 1921. The Omsk cover is franked with imperforate stamps. The last cover is franked with 10 x 1 rouble stamps with horizontal varnish lines.