That looks like pitting caused by arcing caused by a bad connection between the lamp and the lampholder.
Did you screw the lamps in nice and tight? Can you take a photo of the inside of the lampholder? You might need to shine a torch in there if your camera doesn't have a flash.
Did you screw the lamps in nice and tight? Can you take a photo of the inside of the lampholder? You might need to shine a torch in there if your camera doesn't have a flash.