The long tube is a tube sight, normally used in "iron sight only" competition.  Why it would be on the Krag sporter is anyone's guess.   

Tube sights are usually adjusted with an external micrometer adjustable mount.  The front tube mount is one of those, but is in the wrong place.  The aperture in the rear sight has to be in constant alignment with the insert in the front sight.

The Redfield receiver sight appears to be set up as the adjusting component of the tube sight, so that the shooter looks through that in addition to whatever aperture is in the tube.  Good luck with aligning that with the adjustable front mount AND the fixed front sight.

Is the tube sight marked?  Looks sort of like a Fecker.

Is that a Krag barrel?