If you don't have the book I can type up some quotes.
Boyd is clearly affirming omniscience in an
open view way in the answer. However, there is no mention of (a form of) the word possible (or related concept) in this correspondence. Therefore, I would conclude, simply based on the book in question that Boyd had not yet come to the "possibility twist" on on the
open view by the date on this correspondence: April 29, 1989.
Be careful, lest in fighting the dragon you become the dragon. - Nietzsche