Kutter, while it certainly could be used to seat .410 wads, if that were the intended purpose of it why would it have a hollowed out end? That is the feature that makes me think it was a m/l rifle (or pistol) starter.
I made short starters when I competed in m/l roundball competition in the NMLRA. Many had a very short stub that seated the ball just below the muzzle, and also had about a four inch stub, at right angle to the shortie, that pushed it on in. We called the tool a short starter, even though it had two seating stubs. Then, the loading rod, with a muzzle protector, would be used to seat the ball firmly on the powder.
With my serious bench rest and cross-sticks guns I used a ball and patch combination that was so tight I had to "hammer" the ball into the barrel with a mallet on the short starter, and the long starter as you call it. I have a 14 lb. cross-sticks gun, .45 caliber, that will shoot sub-inch 5 shot groups at 100 yards. My best is a 5/8", 5 shot, at 100. Although they are called round ball rifles, when you seat as tight a fitting ball/patch combination as I used in that rifle the ball is actually swaged into a slightly elongated shape. The weave from the patch will be easily seen in the soft lead ball. Very tight.
