it is based off a weapon from the 19th century produced in polynesia made from very long hack blad,
these have a hook on the back for hanging the item and for moving chopped materials and pulling brush out of the way... the same style of knife is still very popular in the apls with or without the hook the cutitng edge is not on the hooked side and the hook is not a broad beak like a billhook but more like a steel rod..
swiss arm used to issue them both with the hook and without..
interestingly i saw one the other day in decent condition sell for 100$
http://cgi.ebay.ca/ws/eBayISAPI.dll?...T#ht_500wt_922