Here’s an interesting tidbit..back in the 80’s I had a 73 Cutlass Supreme.
I was wondering the same thing. While thumbing through the owner’s manual, I came across the gear selector. Turns out..this is AWESOME..the S stands for ‘SUPER’ gear. When super acceleration is needed, put the selector in S for SUPER Acceleration! I almost couldn’t believe it. Pretty funny. So, I’d like to get my hands on a Pontiac manual and see what it says there. Be a safe bet it’s for ‘SUPER’