Guys,
I make my own barrels without using modern machine tools. Round barrels are more of a challenge than octagon barrels. Starting with the forged iron surface i file an octagon (8 sides) to a 16 side shape to a 32 side shape to a 64 side shape. This is followed by a final polish. So, a period barrel often will tend to have the feature as you have found. When doing a gunsmith show-n-tell I let visitors rotate a "round" barrel in their hands as they discover that they are not perfectly round at all but actually a series of tiny flats as your photo shows.
As an aside, I let the visitors sight down the flats of a hand made octagon barrel, not perfect either.
Jim