The square lid consists of two triangular parts, so that the opening is done by lifting one of the parts, and the covers usually do not come separately but in groups of 2 or 3 that form a rectangle.
There is also a model with the same name, but with a completely different background Click for cover's details
GS - Spheroidal graphite cast iron - a special cast iron in which the crystallization of carbon has been slowed down, which makes it close to steel - also called ductile iron (Thanks to Vuori Susi for the clarification)