the SLK-800A/900A are clip-ons like a regular hs.
The SLK-800U/900U are the bolt-ons that need mount holes.
I would NOT recommend the SLK series of hs's on the DFI NF2 if you are going to be moving it a lot, unless you dont have a problem with pulling the hs and remounting it each time you move the machine.
At over 400grams, these things WILL rip the ziff socket right off the board if you are not careful.
If you plan on moving them a lot, get an SK-7. I have one on my DFI NF2 and its VERY nice, and not too heavy.
I did see the SLK-800A's on sale at SVC.com for $24.99 last week =)
Travis