Mac OS X 10.6 Server available by phone order
MC588Z/A is the U.S. product identifier for Mac OS X 10.6 Server. It no longer is available from Apple directly. However a quick search of the internet should help you locate who does sell it.
10.6 Server can be very handy for those who need a version of Mac OS X with Rosetta to function on newer Macs with newer operating systems, as this tip explains: