Did you eventually work this one out? There's only six planes on which an S can be written in three-dimensional space, so you should be able to rattle through the options in a minute or two.
1) going across the fretboard, the frets bending one way then the other. Froody.
2) going down the fretboard, the neck bending to the E side and then the G side and then back again. I like the surrealism of this one, the strings at some points hanging over the edge of the neck as it twists back and forth.
3) on a cross section of the neck, going down from the fretboard, no can't make this one work even in a nonsense way.
4) on a cross section of the neck, the fretboard raised at the D string but lowered at the A string. Would definitely want a freshly cut nut for this.
5) looking at the side of the neck, a vertical line dropped from a fret going .... no, this one is just impossible, too.
6) with the bass vertical, looking at the side of the neck, the gentle bow of the fretboard interrupted with a hump and then a dip.