At first blush it looks like it got thrown in the back of the Land Rover on safari and knocked the North end around. Maybe just the light, but there appears to be similar though lesser damage to the right barrel crown as well, around 4:30 / 5 o'clock. Also some damage to the trapezoidal front site block, bottom left, above the 2 o'clock of the right barrel. The line connecting the barrels? Deep rust from improper storage muzzles down in a damp case/environment that couldn't be polished out?
Well, forgive me- been reading A. Conan Doyle again.
Mike