Thanks for the clarification. It seems that these three denominations were issued to two different standards, both with the same silver content, a heavy type in .750 silver and a light type in .868 silver. It strikes me we should explain this on the individual pages for clarity. I'd be grateful if @Trooper8 could look at the following proposed comments.
The silver 5 kopeck of Alexander I were issued in two different standards, 1.037 g in 0.868 silver and 1.200 g in 0.750 silver. Both had the same silver content of 0.900 g.
The 10 kopeck of Alexander I after 1810 were issued in two different standards, 2.073 g in 0.868 silver and 2.400 g in 0.750 silver. Both had the same silver content of 1.800 g.
The 20 kopeck of Alexander I were issued in two different standards, 4.146 g in 0.868 silver and 4.799 g in 0.750 silver. Both had the same silver content of 3.599 g.
If you prefer, we could give the weights in доля (dolya):
The silver 5 kopeck of Alexander I were issued in two different standards, 2341∕125 dolya in 0.868 silver and 27 dolya in 0.750 silver. Both had the same silver content of 201∕4 dolya.
The 10 kopeck of Alexander I after 1810 were issued in two different standards, 4682∕125 dolya in 0.868 silver and 54 dolya in 0.750 silver. Both had the same silver content of 401∕2 dolya.
The 20 kopeck of Alexander I were issued in two different standards, 9339∕125 dolya in 0.868 silver and 108 dolya in 0.750 silver. Both had the same silver content of 81 dolya.
Former Numista referee for banknotes from Ireland, Northern Ireland, Scotland, Wales and Saint Helena.