I reported earlier about old Sovjet stamps on this theme.
But in this post, I'd like to go deeper into a special issue of 1926.
From the 6th till the 9th of August 1926, the 6th international congress of Esperanto-Proletarians was held in Leningrad (now Saint Petersburg).
Except for the russian words "posta and CCCP", all other texts are in Esperanto.
On top of the stamp, you will see the green star (verda stelo), symbol of Esperanto, and "SAT"
Again, SAT is an esperanto abbreviation which stands for : "Sennacieca Asocio Tutmonda"
(world wide association of (the) nationless).
If you take a closer look to the oval, in which the monument of the Constitution is depicted, you will see something strange :
A lots of dots and dashes, ... in Morse-code :
telling us : "Proletaroj cxiulandaj unuigxu!" ... Proletarians of all countries (world wide) unite!
"cx = ĉ; gx = ĝ"
Truly, a stamp that has more to tell us then there is to see at first glance.