FOM: priority and forcing

[Michele Pagani]

I read, I don't remember where, that Georg Kreisel wrote a letter to Godel,
in which he described the just published article by Cohen on continuum
ipothesis. In this letter Kreisel compares the forcing to the priority
constructions used short time before by Friedberg (and Muchnick).
Does someone know the exact text of this letter? Is it published, or however
is it possible to get a copy?
More widely, I'm interested in historic material about the relations between
this two methods (forcing and priority). I would be very gratefully to whom
could give me useful advices.
Michele Pagani.