Bolshevik Russian submarines exercising in harbour of Novorosssiysk according to the Weekly Intelligence Summary (W.I.S.) No. 10 dated 12 March 1921

An item referred to a merchant ship returning from Novorossiysk reporting that two Bolshevik Russian submarines were exercising in the harbour sailing subsequently for an unknown destination. This confirmed the report that Minoga type submarines were transferred from the Caspian Sea to the Black Sea.

Source

The National Archives, Kew Gardens, England CAB-24-121-21