The original AFCK (I think the model number was 800) was a liner locking knife with titanium liners. It came in ATS-34 or M2. I owned a ATS-34 one, because corrosion is a problem if you are in the field and unable to clean your knife for whatever reason. The only problem with the knife was that there was a alot of flex in the handle, and this made the liner lock hard to disengage.
The 806 (same blade and handle but with an AXIS lock) came out a couple of years ago, and as far as I know it is only avalible in D2. I got one last month, and I really like the D2. The handle flex has been eliminated by the stainless steel liners. I was initially worried I might disengaged the AXIS lock by accident while stabbing someone, but having filed down the thumb side of the lock makes this sort of unlikely, as does a consistant forward grip.