To me that expresses the problem in a nutshell...trying to define something by what it is not.
Why not try the better way and define what a traditional knife is. It's a knife that is made with traditional or modern materials, but the basic knife design and mechanism must have existed before 1950.
However, unfortunately this would allow in automatic knives, which is "not traditional" per some mods, but certainly are old well established "traditional" patterns.