Short answers, pulled from the story.
The Transcaucasian Socialist Federative Soviet Republic was a republic of the Soviet Union that existed from 1922 to 1936, comprising Armenia, Azerbaijan, and Georgia. It was created on the 13th of December 1922 and was one of the four founding members of the Soviet Union. It was dissolved upon adoption of the 1936 Soviet Constitution.
The Transcaucasian SFSR was created following a Soviet Red Army invasion of the region in 1920, at the proposal of Vladimir Lenin. Moscow sought to unify the region economically, militarily, and politically to support the Russian Civil War effort, suppress counterrevolutionary activity, rebuild economies, and reduce interethnic tensions among the Armenian, Azerbaijani, and Georgian peoples.
The Transcaucasian SFSR signed the Treaty on the Creation of the Soviet Union on the 30th of December 1922 alongside the Russian SFSR, the Ukrainian SSR, and the Byelorussian SSR. These four republics were the founding members of the USSR.
Tbilisi was the capital of the Transcaucasian Socialist Federative Soviet Republic. Mamia Orakhelashvili, a Georgian Bolshevik leader, served as the first chairman of the republic's Council of People's Commissars.
Abkhazia initially joined the TSFSR through Georgia as a treaty republic on equal footing with the other federation members, following a treaty of alliance signed on the 16th of December 1921. On the 19th of February 1931, its status was downgraded from a full Soviet republic to an Autonomous Soviet Socialist Republic within the Georgian SSR.
The Transcaucasian Federation began issuing its own stamps on the 15th of September 1923, superseding the separate postal issues of Armenia, Azerbaijan, and Georgia on the 1st of October 1923. Due to massive inflation, stamp values ranged from 40,000 to 500,000 Transcaucasian rubles before being reissued in gold kopecks on the 24th of October.