The legend says that the famous knife used in the first movie was added by Sylvester Stallone (Morrell's book doesn't mention any survival knife at all) which - again - was taken from reality.
Hollow handle knives were really used during the Vietnam War, mostly by SOG. Originally born to keep matches dry and supplied to airplane pilots faced with the possibility of being shot down, they were used by SOG because their mission situations were thought similar to downed pilots escaping from the enemy: a struggle to survive while chased by their adversaries.
The hollow handle – other than being a place to hold small objects – could be used to make a spear, which is what Rambo does when he goes hunting during the first movie.
Below on the left, is a knife that was really used in Vietnam.
On the right is the one used in the movie.
They obviously share many features.
