The serious experimentation with telescopically sighted rifles began in the mid 1920s.Initial experimentation began by mounting commercial scopes then military contracted optics upon the m/91 Dragoon.The use of snipers to tie down enemy troop concentrations was not lost on the Soviets in their brief experiences in the First World War.
The Soviet Union took the task of telescopically aimed riflemen very seriously early on.
So orders were placed for several different optical sights from makers such as Zeiss, Busch, Hensoldt and Voightlander to name a few.
The purpose of these scopes was to conduct trials with various mounting systems provided.
This system used a large thumb screw that was attached to the base by use of a small chain attached to the front of the mount and to the thumb screw itself so it would not be lost.
This system was used on rifles assembled at the "Dynamo" shooting facility run by the NKVD at the time and became known as the D-2 system.
In a defensive position, as the Soviets found themselves so often in the early portion of Operation Barbarosa, the sniper served as an observer to enemy movements.