Assassin - The Caesar's Legion assassin sent to kill President Kimball carries a special version which is "non-playable". This prevents the weapon from normally being shown in inventory, e.g. when pickpocketing the assassin. The weapon is otherwise identical to the standard combat knife.
Cass - Rose of Sharon Cassidy carries a special "non-playable" version of the weapon, also otherwise identical to the standard version.
One in good condition can be found stuck in a blackjack table in the Vikki and Vance Casino. Go into the main room, take a right, into the small room, and it should be stabbed in the table (must be stolen).
One can be found in the second floor of a ruined building in Boulder City beside a couple of skeletons. You will also find a service rifle and a Ranger hat in this building, each in very low condition.
One can be located next to a toolbox, first aid box, and a skeleton. All of which is found inside a boat, underwater, at the Virgin River in Zion Canyon. The combat knife is in near perfect condition.