The Russian government was not adverse to having official subdivisions
within minority groups [after all, Russians themselves were officially
subdivided by class].  For example, in the annuals published in
Lithuania on the local ethnogeography, there are 3 official Jewish
religions:  Hasidim, Misnagdim, and Karaites.  [The orphanage organized
by the Vilna Gaon also included the Karaim as a Jewish branch.]  By 1897
there existed a Russophone wealthy community that built the Choral
Synagogues in the large cities, including St. Petersburg.  They used a
bilingual siddur--Russian and Hebrew.  There were Jewish newspapers
published in Russian.  It was a peculiarly Russian Reform movement. 
"Mosaic Law" might be referring to membership in such a synagogue.  
Russia had no tolerance for unofficial splinter groups, especially if
they originated in the Orthodox Church. Evangelical Christian sects were
never tolerated and were treated worse than the Jews.

Jules Levin, Los Angeles