The photograph on the left is a view of the right side of the 91/30. In this view we can see the strait bolt handle with the ball on the end and below that is the trigger and magazine assembly.
The picture on the right is of the rear sight. To adjust this sight for distance, the operator would squeeze the buttons at each end of the bar and move it in a forward direction. The numbers corresponds to 100 meters so that moving the bar to the number 3 would indicate 300 meters.
The Mosin Nagant rifle by Terence W. Lapin
Handbook of military rifle marks 1866-1950 by Richard Hoffman & Noel Schott
Official guide to gunmarks by Robert Balderson
The standard directory of proof marks by Gerhard Wirnsberger