Possible Die Break

It seems like the break is outside the field of strike for the die though....

Yep, small rim Cud

How does a rim Cud occur? Is that a break in the collar?

No actually it is a break in the die. If you think about how the die is shaped, the field is flat and as it reaches the edge of the coin it then drops drop a fraction of a mm and then forms another flat ring around the face of the die. That flat ring is what forms the top surface of the rim. Then at the edge of that ring it then drops down again forming the shank or body of the die. Now if the edge of the dies chips away so that it only chips away that ring area, when the coin is struck there will be a raised lump that only appears on the flat top surface of the rim. That is a rim Cud.

If the collar breaks then you will get a raised lump on the edge of the coin, not the rim. That's a collar break. You don't see collar cracks or collar breaks too often. The collar is a pretty massive piece of metal.