Well, I went to the source and this is what I got back:
Me:
How does one find out when their rifle was manufactured? I bought one in kit form back in 1982.
Lyman staff:
If it was bought in 1982, it was likely made that year. The Great Plains rifle was first cataloged in 1979, so your rifle is a very early model. Through the years, we have not keep large numbers of kits in our inventory, so they usually go it within months of their manufacture date.
Me:
No, it does not, as I know of others who are interested in dating their Great Plains Rifles, both kit and finished rifles. Is there a code in the serial number? Or a record of what year a serial number may belong to?
Lyman staff:
We have limited records for serial number ranges that could give a rough date for newer rifles, but not going back this far. There is no code for dates in the numbers.
And there you have it.