The initial assortment of Machine Robos consisted of small toy robots which transformed into vehicles, aircraft, etc, comparable in size to Matchbox cars. These early MR toys have become known as the "600 series" (being priced at ¥600 each), and were developed through ideas submitted by children, similar to the children's submissions in the Kinnikuman manga series.
Catalogs packaged with the toys contained stories depicting the Machine Robos as beings from the planet Romulos. Romulos was destroyed by the monster-like Devil Invaders from the Casmozone and thus the Machine Robo came to Earth. This premise would later be abandoned in favor of a new storyline originating with the first MR anime Machine Robo: Revenge of Cronos.