WMA is apparently stands for Winchester Military Ammo and is used exclusively by their Oxford, Mississippi, plant.
Which is somewhat odd, given that .40 S&W isn't a military cartridge. My guess is that they load it at the plant between military contracts and simply use the headstamp to designate manufacture location.
But, most companies have a variety of headstamp styles, and have had for many years.
No clue whether there's a rhyme/reason for it or not.