That was not on the helmet during filming. I do believe that is some sort of marker and not intentional weathering.
Anil, I'm not sure that Don Post ever had the Empire helmet at their studio. The photo with the ESB helmet on the Don Post prototype mannequin was at the archives if I am not mistaken.
Oh, and Don Post did have access to a real helmet (not the Empire helmet), including several detailed shots and measurements. That is why the Deluxe has several ILM helmet characteristics, including the dent.