The Russian DP series LMG's used flap lock recesses carved into the walls of the meaty receivers. These recesses appear to be done using a shaper cutting to a relief slot at the bottom of the stroke that serves as a chip breaker. I explained this process years ago to a local arms manufacturer who was intersted in a US Navy contract to produce Berretta pistol replacement slides. He originally wanted to use EDM to form those pockets but that process takes time - lots of time.
A Bridgeport slotter head or a P&W verticle shaper would knock these pockets out quickly. Even a small bench top horizontal shaper can do this work.