With regard the Soviet Union (and it's constituents)
When the Soviet Union was dissolved, Russia as the largest successor state adopted all international obligations of the former USSR, including its membership to the UCC. Consequently, Russia was henceforth considered a member of the UCC (in the 1952 Geneva text) since the date of the adherence of the USSR to that treaty, i.e., since May 27, 1973. The membership of the USSR in the Brussels Convention was equally continued by the Russian Federation as from December 25, 1991.
On June 25, 1993, Russia and Armenia signed a treaty on the mutual protection of copyrights. To clarify the copyright situation amongst the states that had made up the former Soviet Union, the Commonwealth of Independent States (CIS) nations agreed on a cooperation agreement in the field of copyrights on September 24, 1993. This "Moscow agreement" declared that all signatory countries considered themselves bound by the UCC as of the date the USSR had joined and would confirm this state with the UNESCO, which administered the UCC. The treaty also defined that the treaty states would apply the UCC amongst themselves, also for works published before May 27, 1973 if those works had been copyrighted before this date according to the national laws of the successor states. This provision was subject to the rule of the shorter term. The intent of the Moscow agreement was to avoid that older Soviet works became copyrighted in only some of the successor states, but would become part of the public domain in some of the others. The 1993 Moscow agreement entered in force in Russia on May 6, 1995.