good question!  though MB cites them approvingly he seems to think that every reader is familiar with their history (though at the same time suggesting that they've been unfairly neglected and forgotten by Left scholars) and I am still wading through his essay and have not yet followed the trail back to secondary, let alone primary, sources.

his admiration for the Spanish anarchists is expressed in his manifesto/provocation "Listen, Marxist!" in which he excoriates the sectarian Left with vigour and (imho) considerable accuracy;  also in his responses to critics of the essay.  his takedown of industrial-cornucopian socialism and critique of the mature Marx's  obsession with the factory strike me as insightful...

The difference between theory and practise in practise ...