It is South Korean. The North doesn't make or market M16 style stuff that I am aware of. They are all AK, or home grown.
South Korea has a very active arms industry. If you recall, they started making the K2 rifle because Colt wouldn't let them make the M16 under license. Better for S. Korea. And they make the mags for their own rifles as well. Notice it is designed like the early Colt M16 mags, in that the base plate is retained by a spring clip, which you have to depress before sliding off the floor plate. Current US manufactured G.I. mags do not have this feature any more. Heck, this may be a mag from 20 years ago?? I don't read S. Korean.
Had my Korean girlfriend translate the Korean. The middle characters, between M16A1 and 5.56, she can read, but does not know what it means. She thinks it MIGHT be a manufacturer? Phonetically it sounds like "Qug-young" They may have sub contractors just like we do. The bottom line says "Republic of Korea" (which means South Korea).