The sidescreen slot was not available if a spare wheel was fitted because the opening for the spare, above the fuel tank, takes up that space. I do seem to recall that someone mentioned that it had been discontinued on all models so perhaps it became an option.
Morgan, like all car companies, charges a premium for everything that passes through their hands, genuine spare parts and all that rubbish. In many cases the item is identical to the same thing purchased for half the price on the open market. In the case of the Anniversary steering wheel the brass makes it a special order on Moto-Lita and it is probably exclusive to Morgan.