I think with the right roller press you would simply remove the mag guts, slide the magazine on the same form you used to make them and roll the ribs onto each side and throw them in a pile for reassembly. Heck, it was probably the same process as making the mag, except that the ribs were rolled into the square mag form before the feed lip plate was installed. That would be quicker than having to stop the roller.
TSMG, I believe I have seen some mags with Steyr stempels too, on ayf mags. WaA387 is also found on MP.40 mag components, followers I believe. That stempel may be for one of the other contractors. I think it is. Given the foregoing, it would make perfect sense that a mag maker would already have the equipment necessary to add ribs to mag bodies which would be a simple process, simpler and cheaper than making a new mag as you skip all the other processes and no additional material costs.