A very early US inside primed case. The version earlier did not have the extra fold in the head at the corner (shown in Rays photo) where the rim meets with the body, it was just "L" shaped, As to how hard to find - yes, uncommon, but by no means rare or even scarce. Developed in the early 1870's & mostly (all?) made by the US government arsenals for the military.
Somewhat short lived and predates the Benet primed case system.