The S/C Calmini cat-back has an extra member. It's kinda like an extension piece. The stock S/C and non-S/C are different, so this extra extension makes up for the difference.

It was hell getting off my stock muffler; It broke free, but it would not pull out because of the indentation that that stock U clamp made. I had to cut a slit in the stock pipe and then it pulled free.