There seems to be a minor disagreement going on. In order to settle it, in the following how it was according to regulations, edict, assay charters etc. as from 1711. But before that, silver fineness used in the 18th cent should not be compared with fineness used in the 20th cent. The time gap is up to 150 years and that is quite a long time. A lot of water has flown in river Jordan during those years....
Anyway, the first edict ordinates by Peter the Great in February 13, 1700. It establishes four silver "qualities". However, it is a bit unclear how they were defined. The edict of December 11, 1711 is clearer. It introduces three silver "standards:
1. Melted silver free of alloys (~99)
2. Yephimka, i.e. 82 zolotniks
3. Levkovoie, i.e. 62 zolotniks
In January 24, 1729 a new standard is implemented but only for monetary silver. It is 77 zolotniks. The levkovoie standard (62) is raised to 72 zolotniks. Later the 77 zol. will be substituted with 72 zolotniks. The standards mentioned above will be substituted to a minimum standard of 84 zolotniks in May 1, 1798. In March 31, 1847 a new edict is released. The silver standards are as from that date;still minimum 84, 88, 91. In addition, 94 for drawn or pressed silver and 94-96 for braid silver (whatever the two last ones actually mean?). These official standards are valid in imperial Russia until 1917. What happens after that is another story.
Of course it was not forbidden to use other unofficial silver fineness in Russia but not less than 84 zolotniks.
As to what is stated in Postnikova. It is the best book available for the moment but unfortunately it contains some "inefficiencies and frankly said errors too. I have registered close to 200 "inefficiencies etc.". Juke mentions #10 (76). It could be a mistake or a gold fineness. The gold fineness in 1718 was 75 zolotniks. The dating might be incorrect also. The official standards are still the above mentioned. Please note that the coin standards for both silver and gold differ a bit from the above mentioned standards, but coin standards should not be mixed with this.
The history is actually longer, but I cannot write a whole novel here...;-)