SL 66 cameras were sequentially numbered from 2 900 000 - 2 927 800 between 1966 - 1982. Coded numbers from 7xx xxx xxx were used since 1982.
Rolleiflex SL 66
Coded serial numbers of 9 digits are used for Rollei 35, Rolleiflex SL 66 and Rolleiflex 6000 System. Take 004 230 028 for instance. The first digit plus 5 (the key) represents the year of manufacture, in this example 0 + 5 = 5. One has to guess the decade and the century! Knowing that this number belongs to an SL 66 E manufactured between 1982 and 1986, the year of production is 1985.
The digits placed in the second and third place, (in our example 04) are for factory internal use and represent the engineering level. They are called the Index.
The digits in 4th and 5th place minus 20 give the calendar week in the year of production. In our example the digits 4 and 5 are 23. 23 minus 20 is 3, so the camera was manufactured in the third week of 1985.
The digits 6 to 9 stand for the sequential number of a camera built in that week. In our example 0028 means the camera was number 28 manufactured in the third week of 1985 with an engineering level of 04.