There are various design of similar kind magazine pocket, as far as I know, there are as many as 8 different designs. Most of them are design for SMG magazines, such like M3 or Sweden K sub-machine gun, and a few can handle AK mags.
There is a thing in common for all these designs: ALL OF THEM ARE NOT LIKE USGI STYLE GEAR. Some of them just like Russian style, while the rest resemble Germany. It’s a way to confuse enemy. Most them were made in Japan in 1960s, and none of them has assigned a known stock number.