I'd guess the hole may measure a lighter charge than the charger normally delivers... perhaps for a smaller caliber rifle, so he could use it on more than one gun. He could see a grain or two of powder come out when filling and it reached that point, so he'd know he had that size charge. When loading his larger fowler or gun, he would keep his finger over the hole while filling the charger and emptying it... which he had to do anyway to hold it.
Shelby Gallien