The ring of my EK has "25", ive read it stands for " Arbeitsgemeinschaft der Graveur-, Gold und Silberschmeide-Innungen, Hanau " .
There's also a mark on the Merit cross' ring, but i cant make it out, maybe "800" .... , its a bit faint.
I'm not really familiar with the black light method, will read up on it.
Thank You for the tips!