Doc:  It looks like the screw holding that ugly William's rear sight on your barrel could be the rear screw for the 1901 sight.  This would be a good thing.  All Krag sight screws are an unusual 30 threads per inch.  Hopefully the threads in your barrel's sight screw holes have not been damaged.  You should try removing the William's sight and carefully try that buggered up screw in the rear hole.  If the original sight holes and threads are intact, it makes the rear sight job a cinch!