The spine of the dispenser is the ridge running along the backside of the dispenser. It's given that name because on humans, this is where the spine runs. The spine is the shape of the seam of the two halves of the dispenser.
On vintage models, this is shaped exactly like the tie; with the change of the shape in 1987 the spine was made slightly larger. This difference reveals whether a patent 3.942.683 dispenser is vintage, or is in fact a fake, a modern dispenser with it's feet sanded off. It can be observed when looking at the flag side of the dispenser, or by looking at the bottom of the dispenser. For both methods a wholy symmetrical dispenser is a vintage one, while an asymetrical dispenser is modern.