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because it was mentioned in the last thread.
Its not really complicated to make plastic pieces (guns in this case, but anything really) with 2 sorts of plastic.
You just need two molds with the second one being installed on a injection molding machine running on "semi automatic" mode with a worker (or a robot) putting in the sprues from the first machine (fully automatic) after each shot.
Every somewhat decent tool maker can make tools to work in this way, and literally every single factory hall with at least two injection molding machines and a single worker can produce these pieces.