Are you interested in testing the hardware or software aspects? MVS (Now known as OS390) series are interesting beast. It is IBM's unix operating system/board (similar to AS400) with OS/2 daughterboard stuck on it, that handles various aspects. (one of them is tcp/ip and network)
This is one confusing ugly box that makes me say "What the hell were they thinking?". Farmiliarty with OS2 (experience with REX would probably help), and AS400 would help. Though some of their application behave completely differently then AS400. (Try to configure DB2 to work on this baby and you'll pull your hair out).
Oh, the good ol' OS2. I miss that OS. I even attended their 2.0 kick-off meeting in NY in 1990 or 1991. Good ol' os/2... Make me want to cry... (I probably have a stack of floppy in a box somewhere from the beta program...)