МЕЛЬХИОР in Russian is an Alloy of copper, nickel, and zinc and it is called Cupronickel. Sometimes called Nickel Silver or German Silver, the usual formulation is 60% copper, 20% nickel and 20% zinc.
There are a few different maker’s marks for the Kolchugin plant, specific to certain time periods (see the last photo). This helped to narrow down the manufacturing period.