Something I didn't mention in that comment was how the slider was installed with the binding knob on the wrong side (same side as the elevation knob).  Madsenshooter went ahead and posted that nugget, LOL.  This guy also gave away a (very suspicious) 1896C rear sight - sans elevation slide.  Said he wanted to help out fellow Krag enthusiasts by providing a rare part to someone who needed it for free.  Madsenshooter also told me that the guy claims to possess a RR carbine, although it does not have the correct cartouche. 

My suspicion is that he sold an unsuspecting... and unknowledgeable person the sight by claiming it to be a 1901 sight knowing it was wrong.  When I said something, his ruse might have been revealed which prompted his responses.