Haven't the islands in question always borne Japanese place-names? Itorofu, Kunashiri and so on?
The southern half of Sakhalin was ceded by Russia to Japan as settlement of the Russo-Japanese War. The Russian reclaimation of Sakhalin plus the Kurile Islands after World War II was disingeneous because those south of the Etorofu Straight were never part of the disputed/swapped territory either after the Russo-Japanese War nor in the 50 years or so of territorial disputes leading up to it.