The advantage of using a small piece of brass (with the aforementioned handle) that the cleaning can be localized to just the affected area & not outside it, which is what may well happen with brass wool or larger tools, because it is hard to apply (for me) just to the affected area, & will perhaps give the area around the ‘spot’ a different patina. So unless one does the complete case with it, your left with two different colored areas.
To my eye I’d rather see a dark stain on a case with original patina, than one cleaned. Plus cleaning sometimes / often removes things like the annealing color or tool marks.
Dave notes that being gentle with the extra fine bronze wool leaves this patina.
All that said, it’s your round(s) & however it looks good to you, works for me.
As to the cardboard used in lining the shelves I don’t think it is the case of the powder breakdown. treshkin is correct, acid paper will stain the brass if it absorbs moisture & quite badly, but it is different kind of corrosion. A finely grained grey colored stuff & hard to remove.
Your right Tony it’s easier to find another 8mm Lebel, unless you want that exact lot, date, metal supplier & case maker… So dig out that case in the basement!
I have a once nice .303", headstamped " C.T 7.70 (over) A.PX . 17 " that the powder went south in & split the neck. That one will be very hard to replace. So powder dumped & such.