The SA1 (also labelled RF5A123) is an enhancement chip for the Super Famicom/Super Nintendo.
It is based on the 65C816, which the Super Famicom's main CPU (the Ricoh 5A22) is also based on. It seems quite likely that Ricoh was the designer and manufacturer of the SA1, but I can't find any information about that.
The chip has a Nintendo logo on it, but that doesn't mean it was designed or manufactured by Nintendo. Is there any information on its origin? Perhaps a die shot showing a logo or name?