Thank you very much! Before I posted here I tried to find the hallmark myself and even looked up Schweinfurt, but only found a 12Lot-hallmark, which - of course - looks totaly different. Thank you!
I did not buy the item yet. It's a plain napkin ring. The shape of the hallmarks made me think: "Maaaybe late 18. cent", which would be very early for a napkin ring. Now I know its 1st half of the 19. century, the "normal" time for such an item.