So I have this 590 pump tri-rail (7shot) 18.5in barrel. but when i look up the model number 50778, a non tri-rail version is the only one on their website. the box label even says "tri-rail".
it feels way better than the MAKO tri-rail that Mossberg originally used for their 500s. I actually ordered one separate for my 590/18.5inch(7shot) from their website. I personally think the Mossberg version looks better.