The masters of mayhem were certainly the weapons designers at places like Aston House and other SOE stations. These two "hatpins" are wicked, deadly needles of steel with triangular grooved blades. The triangular shape provides great rigidity in small sizes. It also creates a hole that is difficult for the body to heal. This is why Triangular bayonet blades were outlawed. It is also why many men pierced by a small sword in a duel died of infection from the un-healing wound. Today there are similar self-defense weapons available from different makers. They are easily concealed and light to carry. Making one would not be too difficult either. They have no other function than killing, whether in defense or in assassination. If you are caught carrying one the courts may not be sympathetic. Concealed weapons are always viewed as more evil than exposed ones. That is the strange posture of the legal system. But i won't get into the political correctness (or incorrectness) of self-defense or carrying weapons.
You can find out more about me on the "Stories" pages. My hobbies have included training in Japanese martial arts, Kenjutsu, many forms of knife fighting, long range rifles and tactical firearms. I have written several self published books on muzzle-loading firearms, knife-fighting and gas engines and compressors. I am working diligently on my 400+ page F-S book.