Vasteras Micrometer Rear Sight Insert for Swedish Mausers (repro). 

It fits into the empty central area of the original Carl Gustafs rear sight and It is suitable for models M96/38, M63, M96, M41, etc.

To install it simply remove the large screw from the underside of this accessory, insert it into the leaf and secure it by reinstalling the screw.

The adjustment wheel is graduated with the numbers 3-4-5-6 indicating target shifts of one hundred meters, with dashes that shift the target by 50 meters and with dots indicating shifts of 25 meters. Obviously the intermediate values are easily calculable.

By learning to know the drift of the point of impact in the ammunition firing sequence, it is possible to do small adjustments of the wheel at every "n" shots to keep the weapon always aimed at the same point.