Jason:  Most gunsmiths probably have a peep aperature to fit your Redfield sight;  it should cost around $10.  Recently someone asked on the forum about spare parts for possible breakages.  The universal response?  No one seemed to recall a part failing!  I would suggest that maybe a spare Striker (firing-pin tip) would be good insurance.  I have seen a couple of broken ones, probably from dry firing on an empty chamber.  You might want to acquire the original parts missing from your Krag Carbine.  Just to have them.  (Magazine-cutoff, side-plate screw, butt plate, and an 1896 Rifle or repro Carbine rear sight).  As 'reincarnated' suggested,  Poyer's "The American Krag Rifle and Carbine" would be a useful acquisition and education for $20.  I'm attaching a photo of a #102 Redfield sight, which I think is the model you have on your Grandfather's Krag.

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