A prototype (that some rumors stated was made by Bandai) that was never marketed probably because of price and battery drainage problems). Maybe somewhere in Nintendo laboratories there are some unofficial prototypes. The system was very big, approx. like a Powerbook or another portable computer (see the Super Famicom cartridge inserted in the system to evaluate the dimensions!).
Chris Barker of Superfami.com told me this prototype was produced by Nintendo and shown once at Shoshinkai (Nintendo own trade show, now known as Nintendo Space World).

SUPER FAMICOM PORTABLE