Mike,

The "B" on the side of the rear sight is a "Battle range" mark for 375 yards which was considered a commonly used distance setting for a typical battle range to shoot at. Nothing more.

Hard to say what the two filled holes were from. Almost looks like someone wanted a sling swivel in the wrong place. Maybe someone else has an idea.

Have you been able to find any factory Krag ammo yet?

Hope you plan to shoot the prize soon!