In the Papal States the silversmith's mark had the form of a lozenge, but only in Rome had letters and numbers, in the other cities had only letters .So it is certain that it is a Roman silversmith.
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The books list only known marks and among them there is only one, S 25 S, that can match the one on your chalice.
S23 .. is not listed. Obviously you can not rule out that there existed an unknown Roman silversmith whose mark begins with S 23 ....
Regards
Amena