I bought an off brand 20 rd M14/M1A magazine. I'm not sure if it is checkmate or not (there are no moarking on the mag I could find and the paperwork I had is long gone). Also I'm not sure if I bought from Brownels but I do use them sometimes so maybe.
Anyway, yeah, crap mag. You can look at the bottom of it and see that it is not done quite right compared to a quality mag. I just bought from SA after that and the quality was excellent (though price was high).