Gustav Kersten's "Modell I", then called the "Strassburger Verschluss", now generally known as "Kersten's double Greener lockup", was never protected by a full-blown DRP, Deutsches Reichspatent! Kersten merely obtained a Deutsches Reichsgebrauchsmuster, sort of "protected design", DRGM Nr.123783 for his best-known invention. According to a 1910 G.Kersten statement this design was already in the public domain in 1910, so everyone was free to copy. Sorry, but the original DRGM descriptions were simply thrown away after the short protection period expired.
G.Kersten at that time was a military armourer, stationed at the then German city of Strassburg. As a German government employed official he was not allowed at that time to patent inventions in his own name or to do any business of his own as making and selling guns. So he had an agreement with the Vierordt company of Kehl, a suburb on the other side of the Rhine, to market his inventions. In 1910 the partnership of Kersten and Vierordt had ended in bitter quarrels.