If I am seeing the picture right then the end of the safety lever is definitely messed up. That being broken would cause the weapon to fire even on safe. You are correct that the magazine case was tweaked to become a 3rd variation case and it is for the reason you noted, a change in ammunition. When this rifle was first built it was calibrated to the Mk VI ammunition, so when the Mk VII became the standard some recalibration and tweaks were required. The magazine case was tweaked and if you look close at the rear sight base and compare it to untouched originals you would see that the ramps for the slider were ground down to recalibrate the sight to work with the Mk VII ammuntion.