There is a FOLK SONG INDEX at Oberlin College - indexing SING OUT
magazine. I do not know if it includes PEOPLE'S SONGS BULLETIN.
http://www.singout.org/sorce-fsi-search.html
Obviously, the 40's folk community learned these songs through
the Almanacs/Seeger et.al. concerts, rallys, political meetings,
and through the recordings issued by ASCH and KEYNOTE - see the
Folkways albums SONGS OF THE SPANISH CIVIL WAR. Notes are available
in PDF files (link given for each album)
http://www.folkways.si.edu/search/spanish-civil-war

My copies of Peoples Songs Bulletin are not accessible at the moment,
perhaps others on the list have them at hand ?????

Best wishes, Thomas.