it's a chassis from a vendor called CROM. gives you a mag catch in the proper location for racing or maintaining your manual of arms between firearms. also a flared magwell. it is designed to take a magpul mossberg 500 SGA stock, so if it can take a magpul stock then the original mossberg bolts right up as well (or anything that will bolt to a mossberg; pistol grips, folders, etc.). CROM also sells a pistol grip/1913 adaptor for it. ps, when i bought it, CROM had subbed-out manufacturing to a cnc place in canada, so no customs/border issues when buying.
i'd started with the chassis version of the pc9, but the CROM chassis should work with any version. nice thing about the chassis version is that i get to keep the optic in the scout position which is good for me. i've also got a blackfeather sga adaptor for the m305 so i can put the mossberg stock on it if i want (back when it was still a viable truck gun).