Switchblades

A puzzle, which I just learned about (see Clayton Cramer & Joseph Olson, Knives and the Second Amendment): Federal law generally bans the manufacture for interstate commerce and transportation and distribution in interstate commerce of switchblade knives, as well as possession of switchblades on federal territory.

It exempts, however, common carriers, the military, and military contractors; and it also exempts possession and transportation on one’s person by one other group (as to switchblades with a blade of three inches or less). A similar law in Maine was modified last year to exempt the same group. What is that favored group?