I just purchased a sporterized Krag in very good condition.  The rifle appears to have been sporterized long ago - the barrel is cut to 24", the receiver is drilled / tapped for a side mounted weaver base, 1903 front sight / 1901 carbine rear sight, etc.... 

One of the modifications appears to be the installation of a peep sight, only the base of which remains.  I cannot tell how it is mounted to the rifle and I'm not sure what type of peep sight it's for.

The base has no markings on any of the visible sides.  There is a single screw hole on one side and there appear to be two indentations from screws rubbing on the mount - one on the top and one near the screw hole.

Here are some pics:

It looks like that base may be for a Lyman #33 or #34 Sight. (The two models were identical except for windage adjustment).

I suspect your two 'dimple' areas are the 'mushroomed' heads of the two mounting screws (or 'plugs' covering the screw heads). I can see two small 'circle' areas in your photo, where there is a contrast in the metal. One has been 'drilled' slightly.

The Lyman #33 & #34 sights show up on Ebay (and gun shows) from time to time (for around $100). The #33 windage adjustment is by friction; The #34 is by Knob (micrometer).

Since your base is firmly attached, if you find one, just switch the top part.

The same sight was also made for the Remington-Lee, Lee-Enfield, and 1903 Springfield, with different bases. I think there may be other Lyman models that used the same 'top'. These sights were discontinued around 1943.