In 1975, K-21 was given to 17th submarine division which was a part of 11th submarine flotilla based in Gremikha.
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The submarine was given to 17th submarine division based in Gremikha in 1969 (17th submarine division became a part of 11th submarine flotilla in 1974).
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The submarine was modernized between November 1971 – September 1973 and given to 17th submarine division of 11th submarine flotilla based in Gremikha in 1975.
The K-27 was assigned to the 17th submarine division, headquartered at Gremikha).