The Lavochkin Design Bureau, which designed every Russian lunar and planetary probe since 1965, began studying a Mars Sample Return Probe in 1970, shortly after its E-8-5 probe returned some sample of the lunar surface.
The probe was called 5NM and was destined to be launched around September 17, 1975 on an N1 (nossitel odin, first launcher) rocket, the same type of rocket that was to have taken Russian cosmonauts to the Moon. The probe itself was to have weighted about 20 tons, which was well within the payload capacity of the rocket.
5NM was to comprise a trans-Mars module, which included components of the failed M-69 probes (two launched, none made out of the Earth's atmosphere) and of the M-71 probes (Mars-2 and Mars-3). This module, weighting 3600 kg, was to provide attitude and control during the trans Mars phase, in addition to course correction and radio communication with the Earth during the landing.
The lander weighted 16 tons and was to consist of four different rockets: the first for landing on Mars, after atmospheric slowing by a deployable aeroshell 11 m wide, the second for Mars take off, the third for Mars orbit insertion and the fourth for trans Earth injection and trajectory control during the trans Earth phase. This fourth stage weighted 750 kg and was based on the 3MV Mars and Venus probes of the sixties. On top of this stage was a 15 kg spherical return capsule which contained up to 200 g of martian soil.
As previously mentioned, the spacecraft was to have been launched on or around September 17, 1975 (the same launch window actually used by the Vikings). It was to have reached Mars more than one year later, on September 22, 1976. On arriving Mars, the trans Mars module and the lander would have separated, and the lander would have proceeded for landing. In three days the lander was to have taken panoramas of the Martian surface to decide where to collect samples (the soil sampler design is not specified). After three days, the lander would have took off the surface and would have entered a martian orbit with periastron at 300 km, and a rotation period of 12 hours. After 302 days in Martian orbit, the engine of the trans Earth stage would have ignited and would have pushed the capsule to Earth. The probe was to enter Earth atmosphere on May 14, 1978, less than three years after launch.
The mission was to have been preceded, on 1973, by a 4NM mission, which was to deliver a Marsokhod martian rover, derived from the heavy Lunokhod of the seventies using the same trans Mars module, lander aeroshell and descent stage.
However, the mission was canceled because of the high risk of failure of the probe during 3 years (I think that no Soviet probe ever lived so long) and of the risk of biocontamination if the reentry capsule broke on entering Earth atmosphere.
The program was revived in 1974 with the name 5M.

Bibliography
Perminov, V. G.: The Difficult Road to Mars; Washington, NASA, pp. 67-70

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