a slender metal sheath containing a sliding receptacle which holds a single-edge razor blade. Once extended it would be a formidable weapon (not that the others mentioned aren't).
I have a box of those at work (don't ask). The standard stanley style utility knife would have a slightly deeper cut than this style. however when closed this type is much harder to identify as a knife (the case is metal so on x-ray it should show mainly as a rectangular piece of metal, not very knife like.)
You can pull the handle out the back end, usually to change the single edge razor it holds. Using it this way would probably be better, but the blade corner would probably tend to snag on things. The angle of the stanley knife would keep it from snagging.
They probably carried a mix of knives in case some were found and turned away at security.
