The Mountaineer might have a slightly higher grind. The Mk VI has a slightly longer swedge, and hence may be "pointier". Both of those are probably subject to variation between the machinists grinding them, and may have changed slightly over time. To complicate matters, I think there have been Mk VI's made with a single guard. But they're very similar, so I would consider the sheath difference first. Also, the later Mk VI sheaths have a removeable retained strap/snap, and are thus ambidextrous, while earlier versions had a strap that was part of the back piece of leather. I have no idea when CRK/Gfeller made the change.