BRS-99 mags are proprietary (specific to model).....they do pop up once in a while on EE, but check if mags have been engraved with serial number that were part of the "kits" that came in, regular unmarked are more rare it seems, it doesnt make
them function any different, just where they came from.
I heard a rumor that the dealers that shorted all the preorders of 2 mags (should have been 4) sold them as "extras" at a premium price until everyone noticed the engraving!!