Not strictly true,easiest way to explain is copy this from the RSOC

The outer edge of a S1 Wheel usually refered to as the beading is also significantly thicker than an RS1600i wheel , this can only really be noticed if you put the two wheels side by side, the reason for this is that the rsi wheel was prone to buckling with the wonderful standard of British roads so the S1 wheels were strengthened in this area.

The more observant of you may also have noticed a difference in the spacing of the RS1600i wheel there can be a large gap between the Ford motorsport lettering & the 9057884 Part number these are generally regarded as early production wheels .