English: Russia RSFSR 1922-07-15 reverse of a censored cover sent registered (registration label 'R Moscow No 391') Moscow to Vienna. Postmarked 'MOCKBA' serial 4, transit Moscow double oval 'MOSCOW EXPEDITSIYA 17.7.22' Three triangles meant censorship (Rossica: Soviet Clandestine Mail Surveillance 1917-1941). Addressed to Russian Bookstore library, Jacob Persky.

Taken from: Rossica Odessa: Clandestine Mail Surveillance, 1920-1940:

In 1919, odd date stamps with three (rarely two) triangles at their base began to appear on mail sent abroad from the new Soviet state. Odessa’s “three triangles” date stamps started the year after. Although censorship was widely suspected, there was no hard proof, so the debate as to their significance continued for many years. However, in a 20 March 1972 letter to a U.S. collector, the Chairman of the All-Union Philatelic Society’s (VOF) Scientific Research and Expertization Council, S. Kristi, had this to say about them:

In the difficult years of the Civil War, the [postal] administration was forced to hire poorly- qualified workers for its post offices on the periphery. On occasion, these workers weren’t even sufficiently literate. So, to streamline mail processing, a variety of symbols started appearing on postmarks. For instance, a thick line with its ends cut slant-wise (in the form of a stretched-out diamond) signified that the letter had come from the field post; three small triangles meant that the letter had undergone censorship, and so forth.

It was not just the three triangles; there were other oddities about the first cancels, too. One was that many of them specified “dispatch office” (EKSPEDITSIYA) but neglected to specify which one. Other, legitimate post marks from dispatch offices either spelled out what function they performed or provided the DO’s number.