Pocket Question

I need pockets. I was looking at all the pockets in Janet Arnold's Patterns of Fashion Vol. 2 for the dresses 1866-1877 and they are pretty much all the same, a pocket that sort of looks like an 18th century pocket but it sewn into the seam of a skirt. Except, that Arnold gives NO explanation about how the slash is sewn to the seam. Anybody here know?

The closest thing I could find online is from Jennifer Rosbrugh's Historical Sewing website that says to finish the slit edges just like 18th century pockets and handsew them in, which makes sense, but I want some corroborating evidence before I do this.