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These two were originally going to evolve by trading for each other, like the Unova bugs.
Gen 1 was dark in some aspects. For example, the revolutionary method to store pokemon as data in a computer had a few issues. Bill himself got fused to a pokemon by accident when he tried to digitally teleport himself from a pod to another.
So it's obvious that the Gen 1 trade evos were supposed to represent a freak teleportation accident. That's why the lines require trading in order to evolve, unlike any other pokemon in the first generation. Golem and Machamp are supposed to be freak accidents as a result of a corruption of the data during transfer, not the natural end of the evolutionary line.
Golem gains actual skin, reptilian features, a dinosaur face and loses the two extra arms. His eyes and mouth have the same expression as Machoke's.
Machamp's skin becomes hard and grey like a rock, loses any reptilian features, his face becomes flatter like Graveler's and he gains two extra arms. His eyes and mouth have the same expression as Graveler's.
There is something similar going on with Gengar and Alakazam. Gengar gains a more corporeal, tangible form and a tail. Alakazam loses a lot of weight and his tail. Meaning that during the transfer, the haunting ghost pokemon possessed the medium/mentalist pokemon and stole some of his life force, but in exchange he gained enhanced psychic powers. Neither of the two look like natural members of their line, especially Alakazam looks sickly and decadent.
A similar thing happens in Gen 2, where trading a pokemon with metal coat corrupts the data and fuses them with the coat, turning them into metal versions of themselves.
Trade evolutions used to make sense and involved teleport corruption or digital mishaps. They weren't just regular evolutions arbitrarily locked behind a trade barrier.
