Originally Posted by
treadigraph
They've got a second ex Pan Am SP which is stored at Mojave - I assume they acquired it for spares...
I don't believe there are that many spares which are unique to the SP rather than being standards of a normal 747-200B of the same era. It was one of the upsides of the SP development programme.
In contrast, the scheme to rebuild and enlarge a normal 747-200B to have the same extended upper deck as the subsequent 747-300 ran into some very considerable cost issues which somehow the Boeing designers of the scheme hadn't foreseen in detail, principally because the 747 control runs are up inside the cabin roof which was being replaced. As the increase in seats was not that much anyway the cost was not really worth it, and Boeing still lost a packet on the limited number actually done.