As mentioned, these rings are so loaded with flaws, that casting is the only reasonable technique that can explain their manufacturing process. In my opinion.
I have to disagree, because you can have flaws in a female die for stamping like in a mold for casting, flaws alone do not lead to a way of manufacture or another.
While I can notice that constant presence of flaws in many rings could suggest they were made by stamping, because to take off flaws from a die for casting is for sure less expensive than from a die for stamping.
