Special ordered cars

I was just curious if there's a way to tell if your car was ordered from a dealership without the build sheet (does the build sheet even tell you?) I noticed 1970 340 Swingers have the "M" on the second digit of the vin for medium class cars, I've never seen a vin on a 1970 340 Swinger with an "S" for special like the Dusters got. Did the "S" mean it was special ordered of just a very high optioned car?