...sometimes they need to be slightly modified to work. The modification consists of a small notch ground into the back of the magazine in order for it to work reliably. I have some that haven't been ground and they work perfectly. I pick them up at gun shows in Phoenix for about $15 to $18. Sometimes a seller will try to get $30 for the ones with the notch, but once you see it, you will understand how easy it is to modify it yourself with a Dremel tool and save about $10.
As far as brand, none that I have purchased ever had any kind of markings. I guess they are just military surplus. I have seen some in Shotgun News that indicate they have an original "Cobray" stamp, but to me that doesn't increase the value. Who cares, as long as they work. (I think just about everything with the name "Cobray" on it is overpriced. $42.50 for a wire brass catcher? $18.50 for a mag loader? $28 for a Nomex suppressor cover? Plus shipping? Harvey, you sure got a pair on you.)