As I understand it, the preinstalled license does not actually count as a purchased copy the OP can reinstall on another computer. The preinstalled copy does not require an AppleID to reinstall, and will not cause Mountain Lion to be listed in the Purchases tab of the App Store. If the OP wants to upgrade another computer that did not come with Mountain Lion, he/she must purchase Mountain Lion for the $19.99 from the Mac App Store.