I don't know what blueing process they use, but checking the rib attach alloy would help point to which process might have been used. If they are soft soldered ribs, it's gotta be rust blued. The matte finish can be from the prep. If they glassbead blast the barrrels before rustblueing, it'll come out matte. If they use a particularly strong rustblue solution, it may etch to a matte finish.