My apologies for replying in English. I know this piece well, as it was originally in my collection. The problem with a loose piece like this is that there is no context: nothing to confirm what looks like a date in the postmark. I have seen so many examples of date errors in postmarks, or as Pavel says a double strike that makes it look like something else, that you really need a complete cover to establish a date without doubts.
Myaskovsky is simply a beginner when it comes to postal history, so he makes beginner's mistakes, and drawing too many conclusions from a piece like this is exactly what I have come to expect from him.
The earliest postmark date that I trust is still 26 August 1920, on a cover from Konstantinovka to Blagoveshchensk. It is franked at the local 30 ruble tariff. I presume the piece Myaskovsky illustrates is from the same period (also a 30R tariff) but the postmark date is too blurred to take very seriously. Also note that the Amur oblast' was still using the Kolchak tariffs as late as April 1920 and probably later.