The Bushy mags you got were apparently aftermarket junk. Bushmaster has been doing themselves a dis-service by selling those aftermarket junk mags if you ask me.
USGI stands for U.S. Govt. Issue and pertains to what is issued to our troops. Mags like Okay Ind., Sanchez, Center Ind., Adventureline, etc. are all USGI AR mags.
The British mags are a good buy (try www.ammoman.com) so pick up some while you can but also be grabbing USGI every chance you get.