"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?