"Yunque" as far as I know has various meanings. One of them is a determined worker, someone who withstands adversity.
However, the other meaning of "yunque" seems to be a type of anvil. All of the links I've found within the socialist context draw a distinction between a "yunque" and a "martillo". The former is seen to be submissive and oppressed, with the latter being the social class who impose their will. See:
http://www.expansion.com/2009/01/29/catalunya/1233263780.htm... and
http://www.luzespiritual.org/francisco-candido-xavier/578-an...
No natural translation suggests itself to me, but hope the info proves helpful.
The translation workplace 
