COMECON means "Council for Mutual Economic Assistance"; it was formed by most of the Soviet-influenced countries.
It was created in 1949, joining the Soviet Union, Bulgaria, Hungary, Poland, Rumania, Czechoslovakia, Mongolia, Cuba and Vietnam. East Germany and Albania joined in 1950, the latter leaving in 1961 and the former in 1990. Comecon was disbaned in 1991.
The flag itself shows a quite interesting logo, with five arrows coming from the center back to the center again, meaning redistribution and “democratic centralism”. The number five has nothing to do with membership, just a way to show the red five pointed star of communism.
This image is based on a image in the Grollier International Encyclopedia published in 1991.
I’d bet that the Comecon flag was 1:2 and bright red, not 2:3 and dark red (like in the original Grollier International Encyclopedia image).