Hi Alex
The Martin Walter s/no was stamped into a stick-on label mounted on the centre inside bulkhead area below the instrument panel.
If you mean chassis no., apart from the metal plate, this was also stamped into the outside of the rt/hand chassis member just behind the front bumper.
According to my ancient and failing memory the S3 had the normal Dormobile double passenger seat, but upholstered in black.
Cheers
Ant