As Peter says, there are two options for ride height to match the two basic ride conditions Morgans are built. Early cars had a significantly compressed rebound spring so ride higher than some of the later cars that had the rebound spring only just compressed. NB the threaded shroud and adjuster for the main spring is only there to ensure that the hub just kisses the rebound spring at static ride height - thus making it possible to achieve this condition for all weights of cars, a heavier car will have the adjuster screwed down more than a light one.
As Peter says, the instructions describe this setting up. I trend to prefer the option that sets the car a bit highter than the latest cars as this allows slightly more bump travel for better ride, the roll stiffness is the same of course in both cases. Best regards PJB.
