In late 1937 or early 1938 the Soviet Union decided to hide small arms production numbers. Since each year the serial numbers started over, the highest serial observed each year was the number of rifles/revolvers/pistols produced, it was a "piece of cake" to deduce the numbers of weapons produced. So, the Soviets started at Tula in 1938 and prefixed serial number blocks with "random" 2 letter prefixes. They followed this procedure at Izhevsk by 1939. The prefixes are not in alphabetical order and no one seems to have the sequence of prefixes. Rifles seem to have up to 4 digits following the prefix and Nagant revolvers usually have 3 digits (a 1940 dated revolver has a 4 digit number after the prefix).
I recently got an early 1938 Nagant with an early digits only serial number and was very happy! They aren't common.
Joe