Hi,
In my opinion your belt buckle is in the empire/ late empire style (in Germany called Biedermeier) and then we are talking about the first half of the 19th century.
The maker's mark of your buckle is I am afraid so terribly rubbed or badly struck that it will be hard to get any information from it. The third mark to the right is the town mark. And my impression of it, is that it has the same pattern or texture as the marks from Kassel. The examples Kumpe and I.K are from the 19th century. The silver standard mark 13 Loth (812/1000) looks like the one your belt buckle, bearing in mind that the proportions seems a bit distorted, as if the photographer has taken the photo from aside.

Best regards/JAKJO