I have a Gustloff-Werke 16 gauge O/U that appears to be the same model as yours, although mine has a straight grip stock. It does not have a model designation at all on it. As Raimey mentioned, this is the same company known variously as BSW, and also as Simson for the largest part of their history. My guess would be the name refers to a firearms merchant who had a few guns specially engraved with that designation to give more prestige perhaps to their firearms store.