It looks like the S300 was made for Sota. More than likely, it's just a rebadged RB300. Rega did this for a number of TT manufacturers. I can't say with certainty, but I doubt there's a significant difference between the two. The main thing you need to be concerned with is the pivot to spindle distance. All Rega arms have a mounting distance of 222 mm, so any model should be a drop in replacement.