ButterBolt:  Nice photo of interior of your 'new' barrel.  That's a good looking 1901 sight and I would expect the lever to release with a light tap of a plastic mallet.  The Buffington (1901) rear sight is fun to use at the Range and was the influence for the eventual 1903 Springfield sight.  The grind-mark around the 'lever screw' (viewing your sight from the bottom) is normal.