I have two ZKK602s that were made in 1989 and the serial numbers are 14746 and 15862 so there doesn't appear to be any relationship between serial number and date of manufacture for them.

However, I also have a ZKK600 that was made in 1976 and the serial number is 7652441255. Is the presence of 76 at the start of this serial number coincidence? Or was there some relationship between serial numbers and dates of manufacture for the earlier models in the ZKK series?