Yes, they have changed a lot.
Up to sometime in the beginning of 60's and 70's there is no logic reason for the serial.
During the 80's the first number indicate the year.
They have had series with saws / models that had full year stamped of and on during the 80's too. In the beginning of the 90's they ran in troubble as it was a single number, as you figured out only works for 10 years, there was a bit of panik and bigger plates with fill years were made.
This is not a rule, it is more of a vage indication.
Their system changed too much and too often.