The gap between the flaps and how the flaps are constructed will probably differ depending on who made the doublet and when the doublet was made. Your pattern looks to show that there is an underflap cover the gaps of the over flap. But I've seen other photos that look the flaps are flush to one another without any visible gap. I would say though that all the flaps are connected independently to allow free-er(?) movement.

I would advise against pockets. It would tempt you to put something in there and that would cause unsightly buldges.