The Ostrekoff Jewels

This one is set at the end of the Russian Revolution, and involves the smuggling out of a family's hereditary jewels. Just by happenstance, the young American hero rescues a woman (a mystery woman) who turns out to be the missing heiress of the jewels. Meanwhile, there are tense aeroplane rides all across Europe, and Wilfred cannot make up his mind between the sly, blonde false claimant and the dark-haired true one.