You found the best of all the RB shotguns.  The grooved receiver ring is the signature feature on the RB shotguns.  Those very early production units have all the lovely features of the sporting rifles, a fully sculptured trigger guard with longer lower tang, the sporting model beach block, and hammer along with a lovely sporting shotgun butt plate.  As time passed Remington began mixing parts from various models until the late stuff was basic military with a shotgun barrel. Great find.