"Trim Grade/Class" VIN decoding?

Hi all, this is a bigger-scale question that I have had for a long time. For our cars (1967-76 as far as I am aware), the second digit of the VIN identifies the car's "trim grade" or class. Can someone explain what this means please? My car (1968 Barracuda) is a "high class" trim grade but is pretty basic in terms of options but I've seen VINs for much faster and nicer cars (340, formula s, etc.) with L or M trim grades. What does this mean? Is it dependent on interior options, or is it assigned by the dealer? My car doesn't have a center console or rear armrests, from what I can tell the fanciest interior option it has is the Burgundy Metallic upper door color. What do you guys think?
What are your VIN numbers The second digit is price class.