A lid that usually comes in pairs, each part consists of two identical triangular parts, lifting the lid is done by raising one of the triangles.
The background of the cover is similar to another cover made by the same manufacturer
GJS-500-7 also called GGG50 marks a strong and heat-resistant type of metal while GJS is a sign for Continuously Cast Spheroidal Graphite Iron - which describes a process in which the metal is continuously fed.