As far as I know the only reliable serial number dating is from around 1962/3 to 1969. Those years the first digit in the serial number was the year it was made. After that came the "K" series 1970 until Bear left Grayling around 1978.
There is some relationship to year by the second letter but I believe that is spotty at best. I think wood colors, stripe and overlays help more in the 70's era, but I'm no expert.