Good luck with finding an answer on this. I've been trying to determine the cause of the same type of anomaly on a 1971-S Proof
Jefferson nickel I discovered a few years ago. It shows a well defined raised anomaly that looks as if it were part of the reverse design element that somehow ended up between the uprights in the second "U" in UNUM. See pic below....

Amazingly, no one can tell me with any certainty what exactly caused it.
On a much sadder note... this extraordinary naked eye visible proof coin anomaly has been reduced to being used as a die marker for the
DDO /
DDR variety which it also exhibits.

Edited by JDRMCB
10/30/2017 07:09 am