Thank you for the welcoming, James. Nice to see you, too.
Btw. you can learn Russian script (also used on Serbian and Bulgarian stamps + some other ex-Soviet's eg. Ukrainian) very easy... some of the letters are same as in the Western... A, E, M, K, O, T... you can learn the rest in about half of an hour.

@ NigelC - is note for the inverted overprint mentioned in the SG catalog? As you have advised me, I have found this control insurance stamp in the Michel's... MI 4C (because it has perforations 12,5)... but didn't find any notes about the inverted overprints. Try to found it also in Soloviev specialized catalog of USSR stamps, but this control stamps is not mentioned at all. However, for other control stamps from that period are mentioned existing of the inverted oveprint examples.
However, a really interesting usage in the early Soviet republic in 1923... didn't expect to be so specific in the way of the usage.