Thought it again.
This mark was used to guarantee front sight being upright. The screw itself could not guarantee that, even if it's made accurately, because the angle also depended on installation. For whatever reason if the barrel need to be removed, and then reinstalled, it could be installed at right angle without any special measuring tool.
If this is correct (I don't know), then the mark on receiver could be placed anywhere, German just picked up the bottom middle position (one fixed position needed to be picked up anyway from manufacturing process point of view). Installing the barrel, measuring the front sight angle, then making a mark on the barrel at the corresponding position.
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