Just as a reference thread on the Tremolierstich (a zigzag by any other name) and assaying methods:
viewtopic.php?f=61&t=48537&p=146661&hilit=assay+zigzag#p146661Additionally, there are some searchable threads relating to Russian controlled/influenced areas (1700's-1800's). Riga pops up and one can find some interesting commentary on the heirachical layered complexity of assay offices within the accountability or reporting system. [One late 1800's assayer with intials R.A. does get mentioned...but this is not his shown mark, occuring within the standardized Russian marks system of the 1840's onward.] The Cyrillic possibility is puzzling.
Just thinking about who might desire this item and could afford it, I keep wondering about military officers serving forward areas in Prussia...would a maker in Riga turn his attention southward to a second market? Would he use a Prussian assay and avoid a Russian office? Would a buyer in Prussia accept something not made within their own borders (as a matter of pride) and go with a reasonable close northern source?