Not real familiar with these, but this is the ventilated shroud part that screwed over the end of the barrel. Has a white bead on it. I assume it is a 12 ga piece, based on the diameter. This appears to be new (old stock) that was in a lot of stuff I purchased when a local sporting goods store closed. I'm guessing it's worth $30 to somebody shipped. I can send pics to an interested party.
