First thing to watch with worsted is it may have an inside and an outside. The outside will be slightly fulled, the inside will have a visible weave. A stiffened canvas insert is basted inside this pocket flap. After the edges are whipped down a lining piece, with all of it's edges turned gets sewn to it. Once that's done the basting can be removed.
Here is more Buckram interfacing basted into the waistcoat front. This is the middle, where it transitions to double-breasted. Period waistcoats and coats often included a folded strip of linen on top of this for use as a button stand. As with the pocket flap this will be lined, but with the same red stuff. The outside of the interfacing gets prick stitched to the worsted to keep it from shifting.