madej is right, it is Warsaw in Poland and the assayer OC is very well known Osip Sosnkovsky 1862-1896. He is also known as Josef Stanislavovich S., marking IS 1852-53 and Ios S. marking IS 1850-1862.
Sosnkovsky never worked in MInsk in Belarus. This is a common misunderstanding related to the Minsk chapter in Postnikova on pages 200-201. It is a complete mess. Most of the names worked in Warsaw. Riedel is also incorrectly placed in Minsk as #1906. Minsk and Warsaw town marks are very similar, both a double headed eagle. This probably misled Postnikova, who knows?
Yes, A·P would be Aleksandr Romanov 1899-1904, AB would be Wyrzykov, Andrzei / Vyrzhikovskij, Aleksandr Kasimirovitch 1904-1910/16(?). Both active in Warsaw the mentioned years (A.Romanov moved to St Petersburg in April 1904 and worked there to 1912. Below Romanov's activity list:
Thank you, I had no doubt about the silversmith who is known and appreciated in Poland. I had no objections to the assayer. I mean if everything with the hallmark is all right because it looks a little uncertain because of the double-beating. Regards