Take a look here for
СиKviewtopic.php?f=46&t=49304СиK mark was identified being a trading house, examples for СиГ shown by Goldstein (and yours) are unidentified.Average niello quality for tourists.
I suspected letter ``и`` in cursive was not a middle name of the maker but ``and``.
Therefore,
СиГ could also be a joint enterprise (two silversmiths) , or another trading house.Those niello souvenirs from Caucasus were very popular and widely distributed even outside Kiev and Caucasus region.