Social Democratic Union 'Proletarian' was a political party organization in Bulgaria 1906-1908. The organization was led by Dimitar Blagoev, who had been expelled from the Narrow Socialists for being an 'anarcho-liberal. 'Proletarian' merged with the Broad Socialists, and would represent the leftwing tendency within that movement from 1909 onwards
References
Social Democratic Union 'Proletarian' Wikipedia(Text) CC BY-SA