50 Mi-6s built by Factory # 23 at Moscow-Fili between 1959 and 1962
The construction numbers used by Factory # 23 are explained as follows: The first digit indicates the year of manufacture (0 for 1960 and so on), the second digit is always a 0 (zero), the third digit is the factory code (3 for factory 23), followed by two digits for the batch number and two digits indicating the number in the batch.
874 Mi-6s built by Factory # 168 at Rostov-na-Donu between 1959 and 1980
Factory # 168 used three different construction numbers systems for the Mi-6 over the years. The first one was used between 1959 and 1969 and is explained as follows: Consisting of seven or eight digits the first one or two digits represent the year of manufacture. They are followed by the figure 68 (indicates Factory # 168; not painted on civil helicopters), two digits for the batch number and two digits indicating the number in the batch.
The second c/n system (sequence construction numbers) was used from 1970 until 1974 and is explained as follows: Consisting of six digits, the first two digits represent the year of manufacture, the next two the batch number and the final two the number in the batch. The factory code is not present within this type of c/n.
The third c/n system was used from 1974 until 1980. There is no explanation for this four digit 'nonsense' construction number.