Yuri Gagarin, Gherman Titov, Andrijan Nicolaev, Pavel Popovitch, Valery Bykovsky and Valentina Tereskova !
These sets were officially signed by the cosmonauts for the Russian agency for foreign trades - KNIGA and represent the only official signing Yuri Gagarin has ever done. Also these covers are the only 100% guarantueed authentic signatures by Gagarin similar to the Apollo insurance covers. Even further these jan 10th 1964 dated covers do exist ONLY signed, so there is not even a real possibility to forge these signatures due to the lack of the matching covers.
Still one of the biggest investment potentials in space autographs - if you compare them with the much more expensive Apollo insurance covers. Mercury 7 items also sell at 3-4x the amount !
Vostok 1-6 KNIGA signed covers