The 830S mark is used in Norway and also sometimes in Denmark and denotes the silver content and that the item is indeed solid silver. So we can see that your mirror is solid 830 parts/1000 silver.
The second mark is the makers mark and after a quick search under the letter S in the Norwegian section, I think I found a match.
Have look at the second mark listed under Saether Gull- og Solvvarefabrikk AS in Oslo (translates to :Sæther's Gold and Silverware factory (AS)) in Oslo...
I also found that Saethers was operating from 1936 to 1963 courtesy of another forum. The information was provided by a contributor who is a very active and well respected contributor here at 925-1000. Thanks and acknowledgement to Qrt.S.
Hopefully Qrt.S will see this post and confirm if the makers attribution is indeed correct.