Possibly one of the most queried stamps of philately

I have seen the query at least six times around the web.
These are some of the responses when I asked. (various authors)
This one had me fooled as a child. Having no country name, I'd mounted it under Bulgaria and didn't repatriate it until 40 years later!
The clue is in the town name at bottom left, Beograd, hence Yugoslavia
Children's week 1954. SG says: "This was not officially issued as an
obligatory tax stamp, but it was on sale at post offices." No
significant CV.
From my faint knowledge of Russian, I understand the
words curved around the top "Dechija Nedel'a" as "Children's Week".
Yugoslavia - postal tax - children's week
Scott RA12a issued 1954 Oct 04
2 dinars brown and salmon
cv $0.60 mint $1.25 used
The letters FNRJ beneath the child's face stands
for Federativna Narodna Republika Jugoslavija
(Federative People's Republic of Yugoslavia)
In 1963, the country's official name changed to
Socijalisticka Federativna Republika Jugoslavija
(Socialist Federative Republic of Yugoslavia)
Yugoslavia means literally "The Southern Slavs"
"yug" = "south".