Ok I've gone through the setup and now at the desktop. During the setup I did not put in my Apple ID for iCloud or anything else, selected Skip, and did not do the registration, again selected Skip.
I'm now going to select the Mac App Store icon in the Dock, App store comes up and now going to sign in. Did that and my name appears in the Quick Links area, Welcome MyName.
Now going to purchases. All are there but the ones that are already installed in my other OS installs, both Lion and Mountain Lion from the previous ML install and copy of my Lion account are listed as Installed.
I am now checking Updates, and it comes back with No updates available.
Going back to Purchases and it is the same. So this is a case for Apple MAS support. So you will need to call them.
I may take the internal drive out if I get a chance and redo the ML install clean again, with no other drives installed or connected, to see what happens then.