That rib is about as beautiful as any engraving work I have ever seen on one. Considering the engraving, I would think this was built for a serious gunner with deep pockets. Can't imagine its use but for long range shooting.
Appears to me to be a range finding mechanism.
Best,
John


Humble member of the League of Extraodinary Gentlemen (LEG). Joined 14 March, 2006. Member #1.