Galaxian (Irem) is a version of the Galaxian arcade game produced by Irem. While the core gameplay would resemble the original Namco version, Irem's iteration might have its own unique aspects in terms of graphics, sounds or other minor details. Irem likely released Galaxian under license or agreement with Namco, although the specifics of the arrangement would need further investigation. 

Galaxian (Irem)

Galaga 3 (set 4)