Duncan, that curved "cover" you refer to is a hoof pick. You use it to clear a stone, or whatever a horse might pick up, causing it to be uncomfortable and limp.
There is a very common feature of these knives though that is apparently not well understood (at least I've never seen a good explanation) -- notice the "scoop" under the hinge end of the hoof pick - it goes all the way across the knife thru all the liners and the scales. You'll see this on many of these knives from many makers. I've even seen people claim that it was for cracking nuts (ridiculous). I've wondered if it was for holding leather leads while working on/with them while repairing tack.
