Rogue Studios ask and tries to answer the question on RMR Slides made for the Marui Glock 18C in their Q&A. The obvious answer is that the Marui Glock 18C and other Marui pistols are cloned by other manufacturers that any third party manufacturer would build parts for the Marui products and confident that it will fit the clones too. Anyway, watch the video below for more insights.