Not trying to be a smart ass , but the newest it could be is 31 years old, to as old as 38 years of age .
Let's just say , old enough to need attention (if it spent any time in a pro's hand) . Peter
I don't believe there are any lists that correlate serial numbers with production date. The 6x7 was introduced in 1969 and started with serial number 4000000 and was replaced by the 67 in 1990 at around serial number 4159850 (give or take around 50). The 67 was replaced by the 67II in 1999 at around serial number 4220000 (give or take 10,000). The 67II has its own set of serial numbers which appear in batches rather than one long sequence and in a couple of cases overlap the 6x7/67 serial numbers. If the production volume was the same all the way through then 1982 would be about the right answer.
I've spoken with a technician at Pentax Service in Hamburg, Germany and he said exactly the same thing. Pentax cameras and all other stuff they made cannot be dated by serial number.