You will see a lot of these in early 60s bags and rolls. I've come across bags that have as many as 10% in them. The quality control was lacking back then. Near as I can figure the copper was not mixed or clean enough when they were rolled. That left pock marks, fissures, and laminations in the finished planchet. The striking force didn't fill in the marks because the planchet was under weight. Just like this is the same area on Lincolns that doesn't fill in with a regular low weigh planchet.