Bakunin's letter on "organizational dualism"?

I was re-reading "The Global Influence of Platformism Today: Italy
11. The Global Influence Of Platformism Today: Interviews"
http://anarchistplatform.wordpress.com/2010/06/14/the-global-influence-of-platformism-today-italy/#more-837

And came across this reference:

"Starting with the excellent studies made by Masini (not by chance one of the most prominent militants in the GAAP) a series of studies were started, above all by our anarchist communist comrade Dada. Her volume “L’anarchismo in Italia: fra movimento e partito” ["Anarchism in Italy: Between Movement and Party"] was a turning point in studies on Italian anarchism. "

Which continued to state that:

"It highlighted not only the communist basis of anarchism but also the original theorization of the principle of “organizational dualism” [1] which had its highest level of theorization in Italy during the First International, from Bakunin to certain correspondents such as Celso Cerretti, to whom Bakunin wrote a letter clarifying this question (republished together with a lot of other material in the book)."

So I am very curious about the Bakunin letter regarding "organizational dualism"

Anyone know the letter in question....and have a link to it (pref. in english, but other roman languages are ok).

Thanks!