Total speculation here, as I've never had one of these apart...
The spring is most likely a large coil spring.
The long hollow tube and the screw location argue strongly for a coil spring rather than a leaf spring, which is the other option.
In such an application, the spring must be stretched along the tube in its relaxed state, because as it's wound/tensioned,
the effect is to add more turns between the end points, and empty space must exist for those turns to occupy.
To accomplish this would require a special fixture/jig to hold the parts in place, to facilitate insertion of the screw.
I suspect a nut or threaded hub is used with the screw, and probably held in place by the same fixture.
The admonition in the manual to leave the screw alone supports the assumption that they used an assembly fixture.