It was based on two solid-fuel fibre-glassclad stages of the RT-21 Temp 2S (SS-16 Sinner), so it was also known as the RT-21M Pioneer. - RSD-10 영위백

The RT-21M Pioneer (SS-20 Saber) and succeeding missile complexes relied on the SS-16(= RT-21) base concept and were used by Nadiradze for many of his later projects. - RT-21 영위백

RT-21은 FSC가 개발했으므로 RSD-10(SS-20)은 FSC것이다 이런 말입니다.