I have them as "Political Vignettes"
There is a vast amount of material existing.
Issued around 1937
I am unable, as yet, to find out anything of them, in my Civil War Catalogue that is over an inch thick. (In Spanish)
Image not mine.
I have seen the 5c (type second from left) on a cover, dated February 15th, 1938
I just found this in my database,
Perhaps you can find in Edifil or Yvert Catalogues?
Please post back if you find information.