This seems fine and accurate to me. It suggests that she was aware of the (claimed) existence of the letters when she was doing her research and was disspointed by her inability at that time to find quotations from them.
While your revised sentence could, I think, suggest that when she was doing her research, she would have found it helpful already to have been aquainted with the letters. It does not, however, necessarily imply that, at that time, she was even aware that such letters or claim existed.
Given just these lines, I have difficulty with the pronoun 'he,' since it is unclear whether its antecedent is "one historian," or some word in a previous sentence. The apparent equation of the letters themselves with quotations from them in the last also gives me trouble. I think, if the sentence were less of a stumbling block generally its tense would be less of one, also.
