Thanks for presenting us with this nifty mystery. I've had the opportunity to look at a great many medieval objects that were not this thing. Not a palstave, darnit.
"British leatherworking tools. Knives from Cairnholly, Cleughhead, Luce Sands, Traprain Law and Camelon. 7500BC-900AD. At least the dates are fairly obvious on this set, even if it does cover nearly 8000 years. The shoe is from Newstead." From.
"A small but significant assemblage of leather-working artefacts have been recovered from the excavations all of which appear to relate to a single workshop area. An iron "lunellum" or crescentic leather-working knife was recovered as well as three pumice rubbers..." From.
Honestly, the leatherworking/cobbler tool guesses are great ones; I've seen tools for pressing design motifs that look rather similar to the artifact in question but alas, it is no such thing. Thanks for the visual lesson Surf, nifty info on shoe-making.
It is not a weapon, and was not used for disemboweling or preparing food. Yes, it is a tool. [8D]
Also, if it helps with your brainstorming, I should mention that I have not included a few other photographs of the item because they would give away the purpose too easily... They will be posted at the conclusion of the contest, though.