Take Grant's descriptions and dates with a grain of salt. He has a lot of pictures, but he tends to be a bit spare with real info in his descriptions, and he was writing many decades ago when folks did not know as much about the development of the American longrifle as we do now. On the other hand, one of the biggest problems in this area is dating artifacts earlier than they really are, usually 19th century objects being taken as 18th century. Grant is actually pretty conservative in that regard - only three of the hundred-plus pouches are claimed as possible 18th century pouches, and you are looking for later pouches anyway. Also, unlike, say, knives, I don't think that there has been any real significant changes in our understanding of pouches since he wrote his books.
All this to say: If you find a pouch you like, cross-reference it with similar pouches from the other two books and see what they have to say too.