Such inscriptions advertizing the suitability for smokeless loads on Belgian guns intended for the German market makes a lot of sense to me. We are constantly answering questions on this forum and others, or by email, like "where, when and for what load was my gun proofed? What does this little lion over PV mean."
Apparently precious few people are able to read proofmarks even today! It was not better in the good old days. And: The Belgian proofmarks, valid in Germany too, did not show a familiar symbol over/under N mark nor the word Nitro, only little lions with cryptic letters like EC, SCH, PT or PV. So some Belgians or German importers felt the need to make it clear to their clientele that the new gun was indeed suitable for modern smokeless loads.
BTW, there was indeed a shotgun proof in Belgiun from 1924 - 1968 called fourfold, the marks were a crowned EL with laurel branches for treble bp proof of the barrels and a crown over PV for a higher than usual smokeless pressure load.
