Thanks all.
I know it is a difficult topic as soon as you go to short stories, or even further, to single quotations. I found two references in Captain Easy (1936-37), the beginning of Three Aces in Action Comics, one story (Bombs of destiny) in Funny Picture stories, one reference in Captain Terror in USA Comics, one reference in Cpt Battle Jr, one story in Spy Hunter. And later, Blackhawk, Wolverine, The Crimson Avenger, The Shadow... all involved in the Spanish Civil War.
Still looking. If you see anything...