If they were created when the planchets were made, I can guarantee that it wouldn't show once the planchet was struck into a coin. The pressure onto the planchets are too much to leave any lines or what not. Those had to be PM marks. Storage wise comes more into play with such a pattern IMO.
It was in a Dansco and the owner had a lot of schmutz on the little plastic thingy. They also had OCD and had to move the plastic cover in and out exactly 5,555 times a day. Thats my answer and I'm sticking to it.
I think most people probably miss the most important part of that quote - And if you go back and read that thread closely, Mike isn't saying those marks on the coin are caused by a descaling brush, he says they may be caused by a descaling brush. Bit of a difference in that.
Found some more looking through Kennedy sets I found some more of these "planchet error" type coins...here are the pics of an 89P and a 73-S
Here are a couple more...a 72-D and a 71-S. View attachment 117648 View attachment 117650 View attachment 117647 View attachment 117649
In those four coins there are Philadelphia, Denver, and San Francisco mint marks and a span of 18 years...has the process been that consistent, across locations, for that long?