Did some more research and according to http://dell.avocent.com/Dell-KVM-Switches/Dell-KVM-Switches.html

there are two part numbers for 1U rail kits: (1) MF257/ (2) 4T334. Anyone know the difference?

(1) MF257- This is the 4 piece bracket kit. Sorry about the other  part ((2) 4T334) but it was lsited wrong. It should be (1) instead and it is the hardware or screws for the brackets. Sorry again and I will change the information correctly on dell.avocent.com.

So MF257 are the rails. What is 4T334 exactly? Do I need it?

It is the hardware packge or nuts and bolts. I am trying to find out more information about 4T334 but I believe that is just the nuts and bolts. You wont need it if youc an supply your own.

The MF257 is all that is required. The other part number is for another Dell branded product. Sorry about the confusion.

Just received the MF257 sent by Dell. FYI: you need to order TWO of those, as it seems that one MF257 is only enough for one side of the switch (two rails which you need to slide into each other). Luckily, I was only missing one side!

Thanks for the good info.