Who is this Duchess?

Having heard nothing from his son for nearly a year and no update from the War Office since his capture on 6th January 1920, Charlie Hayes approached a Duchess, who wrote to the former Senior British Commander in Siberia, Major General Sir Alfred Knox on 20th August 1920 seeking information about Captain Eric Hayes of the Norfolk Regiment, who at that moment was languishing in the Ivanovsky prison in Moscow with fifteen other prisoners-of-war.

In his response, Knox claims to have “done all that I could since my return, to worry high or subordinate officers at the Foreign Office and War Office” and he had “even written to The Times!”  He distances himself from the railway mission, which Churchill had placed under his command, by claiming “only two of the fifteen belonged to my Mission” and limply concludes: “I hope you will use your great influence to get something done by the Government”.

I have not been able to discover the identity of this Duchess.  She is unlikely to be royal, otherwise Knox would have addressed her differently to “Dear Duchess”.  My guess is that this is Gwendolen Mary Fitzalen-Howard, wife of the 15th Duke of Norfolk, a generous philanthropist, but since writing the book Churchill’s Abandoned Prisoners, I haven’t been able to confirm this with corroborating evidence.  Can anyone help?