A scan of the Encyclopedia Brittanica entry appears to bear this out. There does not appear to have been a complete translation of the Old Testament into Old Slavonic by SS. Cyril and Methodius. It appears that missing OT books in the Gennadius version (1499) were translated from the Septuagint and the Vulgate. The Ostrog version of the bible (1581) was corrected even further in the direction of the Codex Alexandrinus Septuagint. There were further revisions (in the direction of the Septuagint) including the definitive 1757 second edition of the “Elizabeth” translation.
The Russian Synodal version is not in Church Slavonic, but in the Russian vernacular. The Old Testament is based on the Masoretic text. The 1876 edition appears to be the mother translation for a number of other translations.