We don't have much data on the serial numbers, but you can still determine the production year by looking inside the piano. There are two 4-digit date codes, one on the pickup rail and one on the tonebar rail. The code on the pickup rail is located on the right-hand side, beneath the part number. The first two digits indicate the week of the year, and the last two indicate the year (e.g. 0978 = 09th week of 1978, sometime in early March). This is the week the pickup rail was completed, which was normally 1 to 4 months prior to the day the piano was finished. This code is the best indicator of the piano's age.
In cases where the pickup rail code is unreadable, you can also look for the finish date code on the lower right-hand side of the tonebar rail. From 1965 to 1973, this also followed the week/year convention mentioned above. Sometime in 1974, the code was switched to week/year/day, where 1581 is interpreted as 15th week, 1978, 1st day of the work week (Monday). When 1980 arrived, the year digit started over at 0, so 1501 would be 15th week, 1980, 1st day of the work week.
As a point of reference, here's an overview of the different models and their production years: