So I was watching James May as the re-assembler on you tube (awesome show) and he said something really interesting, he started trowing all the bolts to an old push mower into a box. Well I was a mechanic and could always identify this while pulling down parts off a car and I'm sure most of you have recognized this with pins. Pinball machines are mostly designed with parts that will only fit one way and aren't interchangeable for the most part, keyed connectors, screw lengths, directional brackets and so on. He said the Japanese have a word for that and its Yuiitsu, but when I researched it it really doesn't fit the description (https://www.quora.com/Is-there-a-Japanese-word-for-it-only-fits-one-way) and its not an singular American word. Ok I'm a '79 baby and consider myself the last year for Gen-X, not trying to be a douche millennial lol but we need this word! All I can think of is Zipper? Remember it has to be American as pinball