Crown of Russian Empire, or the Elusives Again (1970)

The Russian Civil War is over and the White Guard are all dead or abroad. Some of the exiled nobles are attempting to crown a new Tsar/Emperor abroad, and a couple of impostors are vying for the position. At the same time, some of the Elusive Avengers' old foes are trying to return to Russia, for mysterious reasons. The avengers are assigned the task of finding out what's going on. Turns out the bad guys are after the Russian Crown, which is encrusted with many diamonds and jewels. It is not clear whether the crown is only wanted for the coronation ceremony or if the bad guys are trying to sell it to the highest bidder, but either way they must be stopped. The avengers are after them. Two go to Paris to infiltrate the emigrant community, and the other two work on the Russian soil.