Two Fly Pieces and a Metal Wire
One particular "overalls improvement" detail I have never seen used on an actual jean is that of Austin Clement's 1880 patent, which was granted for an invention that connects the fly pieces together using a wire link. It is quite interesting and I would love to see it being employed.
If you are interested in my further ramblings on this matter then I suggest you study the patent illustrations above before continuing. I am about to sound very confusing.
The resulting left and right fly pieces are each double layered and very well may have four layers of denim, which is the design's greatest downfall. The extensive use of overlocking raw edges can work around this issue and make each side only two layers thick but that can create a less clean looking interior and a less practical fly piece. Yet I am sure this design can be feasible with some thought.