Hi, that's not a hallmark (which is an official mark on precious metals) but perhaps a brand logo and manufacturer's ref. number, probably for the design.
The buckle design is art nouveau in style, but I suspect the buckle is not very old at all and made in an antique style - you can see the base metal underneath the worn parts.
This type of fastening on a belt buckle is one I have seen within the last 10 years on wide belts intended for the hips rather than the waist. The silver metal blob on the back probably had a ball ended prong on it that has broken off - this would go through the holes in a leather belt.