Very interesting case.
Austria - Hungary marks compared - yours is partially struck.In the middle number 13 is expected (fineness expressed as 13 Loth).
I have never seen StGeorge,very similar to Moscow assay office,on Austrian silver.On the other hand, there are examples of ``Russified`` silver,i.e.faked Russian marks added to already existing silver in order to increase the value.But this StGeorge mark is done with extremely well done and detailed punch, never seen in faked marks.
The style of the plate is typical Vienna Biedermeier rose silver (Wiener Biedermeier Rosensilber).
In a word,StGeorge is a mystery to me.