Bishop stocks are very common to Krag sporters (my sporter has one too).  24 inch barrels likewise are a common cutoff length (again, my sporter is also 24 inches).  My guess is that the dovetail was cut where the other rear sight screw was.  You might consider a Redfield No-Drill 102K rear sight.  They are common, reasonably priced, easily installed, and quite accurate.  You can usually find them on Ebay, Gunbroker, and other sites.  ButlersRangers can talk more about the Redfield sights.