It appears to have been modified. I think I can see the flaps of the collapsing hood below that plastic overlay that has been screwed on. The finish of the overlay does not look like Mamiya made it to me, but you never know. It is not in my Mamiya System Handbook, but that is an older book, written when the RB was a pretty new camera. I'd guess that it was a generic brand aftermarket piece. It probably blocks a little stray light entering from the sides and makes it more comfortable to rest your face on the finder by covering the metal corners. I find myself pressing my brow against the finder pretty regularly when shooting. The rigid "chimney" magnifying finder for my C series cameras is my favorite WLF I have ever used. It is my standard finder for those cameras.