I have, unfortunately, seen all too many backsights like this in Germany. A couple of decades ago, some bureaucratic geniuses with the all-too-common detachment from reality of those who formulate gun control laws, created the "Kriegswaffenkontrollgesetz". One of the requirements was that bayonet bosses had to be removed and sights had to be limited to a maximum range of 300 meters.
Maybe the unsung heroes who though this up imagined that there would otherwise be armies of loonies taking potshots at, say, 1500 yards, and then charging the rest of the way with fixed bayonets. This led to large numbers of Lee Enfields and Garands with ground-off bayonet bosses(which, on a MK4, means that the barrel is FUBARed for collectors). And sights limited by a welding spot. Garands were also limited to take only 5-round clips. These idiotic requirements have now been deleted.
So the gun is perhaps an import from Germany. Either buy a new backsight or (my tightwad solution) spend a creative evening with a diamond file and file the blasted thing off! Fortunately, it appears that many gunsmiths didn't like spoiling the weapons either, so the spot is often made so that a fully functioning sight can be recovered.