This is one of few special fx knives used in the film.  
This particular knife is made entirely out of
aluminum. There is a small pin hole that has been
drilled all the way through it, tip to tip.  This was
made this way so that it can be slid  on a zip line to
the target and hit where intended.  One end of the
line was off camera, the other attached to the actors
with a piece of balsa wood for the knife to stick in.  
You can see a small amount of tar or plumbers putty
still on the tip.  This putty allowed for the knives to
really stick.