I'm wondering if something is missing from the left barrel of the insert (in German called or Einstecklauf or just EL). You can see on the other EL the 4 adjusting screws at the muzzle. But this piece seems to be missing on the left barrel. Some EL were shorter and there are a number of different brands and designs, but the left barrel still looks odd.

The muzzle adjustable EL, however, is the best one to have because it is easier to adjust. Believe it or not, they are supposed to adjust their point of impact by bending the barrel. They are noted for accuracy, so perhaps not entirely a silly idea.