Initial production variant. Later replaced in Yankee-class boats by Mod 3.
SS-N-6 Serb Mod 2
The Mod 2 was an upgrade that used more powerful engines and an improved guidance system. In Russian service, the missile was designated R-27U. Although designed to carry both single and multiple warheads, the Mod 2 carried only single devices. The upgrade to the missile was authorized on June 10, 1971.
SS-N-6 Serb Mod 3
Introduced MRV warhead to Soviet SLBMs. There was some debate as to the number of RVs carried, either two 600-lb RVs or three 400-lb RVs. The existence of a two-RV Mod 3 variant has not been confirmed by Russian sources.