The thing is that these things are called 'entrenching tools', searching for that gives more results. See e.g. http://en.wikipedia.org/wiki/Entrenching_tool - the pictures there don't look like they have serrated edges to serve as a saw, but there are plenty on e.g. Ebay that do. I know I used to sell the folding model, with serrated edge, 15 years ago in the military surplus stored I worked at.