Based on s/n, it sounds like one of the "Made in Japan" remakes. A picture would help. Depending on vintage, an original would be numbered 3XXXX for a pre A model, 4XXXXA for an A model, 4XXXXB to 7XXXXB for a B model, and 8XXXXC to 9XXXXC C model range. The original sporters were made from 1934 to 1960 at which point the Sporter model was discontinued.
Steve