I recently got some IBM Xseries 335 servers 2nd hand, and noticed they don't have any graphics card. Some google searched indicate they only have an uplink port, which you could connect to another (other model) IBM server and use that as the head node. But, I don't have one available.
That'd be the C2T (Cable Chaining) technology they were using back then. It's essentially a KVM switch system built into the server, the idea being that if you have a rack of 42 of them you can hookup up a single monitor/keyboard/mouse to the stack and with the press of a button switch between the servers.
All you need to get access to the console on these units is a C2T Breakout cable. It looks like a custom KVM cable with PS2/VGA ports on one end and a DVI port on the other.