Here is a You tube link to Pete Seeger performing the XV song with his grandson Tao in which he says the song was taught to him by friends in the Lincoln Battalion, but is it conclusive.

Pete Seeger

I was hoping that someone would state conclusively the year in which the Seeger/Asch recording was issued or made because the difference in the 1940/1943 dates given above means that Pete Seeger or a brigader friend either had a few months or a few years to originate this version of the song if it had not been sung while the war was being fought. (the International Brigades were withdrawn from combat in September 1938 and then repatriated before the war ended in April 1939)

La Lutta Continua

Geoff