I see you have a jacketed bullet in the photo. For what it is worth, the proof inscription says Bl.G meaning Blei-Geschoss or lead bullet. This cartridge was made in both lead or jacketed options. Perhaps the message from the proof marks is that a cast bullet similar to the original factory one, in shape, weight and speed, might help all those leaf sights regulate! (and perhaps be kinder to the rifling).

The triangles on the flats are the factory mark, based on the mountains around Suhl.