Wow, that's a neat car!
Baltimore-built cars didn't put many codes on the firewall tag.
My 65 only has an 'S' on it too, but it barely had any options....just 4 speed, radio, back-up lights and door edge guards.
There's a good chance the car has a Fisher Body build-sheet stuck in the rear seat springs that contains more info.
The protecto-plate would have a few option codes on it too.