This system is descended from the later Russian Empire, but its first (caliber) and third (unique identifier) components were changed several times over the years.
The first component of the designation was caliber in inches or lines, then type of a piece with optional producer's name and attributes such as regimental, divisional, siege, field, fortress, etc.
(6-inch fortress howitzer Model of 1909) and Canon de 42‴ Schneider was designated 42-линейная полевая тяжёлая пушка образца 1910 года (42-line heavy field gun M1910).
The industrial growth of the Soviet Union during the late 1920s and the 1930s allowed the modernization of older or obsolescent artillery.
Modernized ordnance received a second model designation after a slash, for example 152-мм пушка обр.
(122 mm howitzer M1938), but another one was the index of a factory (or a developer: ordnance plants in the Soviet Union very often had their own design bureaus).
During the World War II some indices changed: The artillery sharashka OTB of NKVD (in 1938—1941 and 1944—1953 it was located in Kresty Prison complex in Leningrad, in evacuation it was housed in the buildings of Plant No.
For some pieces of artillery, both the Army and developer names were well known and interchangeable, such as the 76 mm divisional gun M1942 known also as ZIS-3.
[1] The third change was connected with introduction of the GRAU system of indices for ordnance, weapons, and munitions.