There are several pictures that can be found of this oddity used in the Vietnam war. It is an Ak mag welded or attached some how to a USGI 20 round M16 magazine. This resulted in a weird hybrid that may or may not have worked reliably. It seems the vast majority of these oddballs are seen in pictured being used by ARVN troopers. Most people tend to agree they were a local made thing. No one really has any idea or facts on these mags that I have been able to find trustworthy. Post war some company or other made some a little but more refined here in the US with reports of their quality being hit or miss.
Finding out about these has been something I have revisited on and off for 2 decades. I have seen photos online and in a couple of books but still have nothing I would offer up as solid facts on them.