Best Casting is a brand name of a series of manhole covers at the manufacturer NATIONAL FOND, and according to this the identity of the manufacturer was determined.
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)
The lid is similar to another manhole cover made by the same manufacturer