So this might be tricky, but seeing as there are different laws about carrying knifes, it might be a good idea to start to collect laws by jurisdictions, so that users do not end up wanting what they cannot legal have.
Whoever manufactures or causes to be manufactured, or sells or exposes for sale, an instrument or weapon of the kind usually known as a dirk knife, a switch knife or any knife having an automatic spring release device by which the blade is released from the handle, having a blade of over one and one-half inches or a device or case which enables a knife with a locking blade to be drawn at a locked position, any ballistic knife, or any knife with a detachable blade capable of being propelled by any mechanism, slung shot, sling shot, bean blower, sword cane, pistol cane, bludgeon, blackjack, nunchaku, zoobow, also known as klackers or kung fu sticks, or any similar weapon consisting of two sticks of wood, plastic or metal connected at one end by a length of rope, chain, wire or leather, a shuriken or any similar pointed starlike object intended to injure a person when thrown, or a manrikigusari or similar length of chain having weighted ends; or metallic knuckles or knuckles of any other substance which could be put to the same use and with the same or similar effect as metallic knuckles, shall be punished by a fine of not less than fifty nor more than one thousand dollars or by imprisonment for not more than six months; provided, however, that sling shots may be manufactured and sold to clubs or associations conducting sporting events where such sling shots are used.