Isaacs's 1965 preface says he first saw Pontryagin's book just days after finishing his own manuscript in March 1963

rufus-isaacslev-pontryaginmaximum-principledifferential-gamesmultiple-discoverypriority

In the preface to Differential Games (1965), Rufus Isaacs gives his own first-hand account of his near-miss on priority against the Soviet maximum-principle school: "In fact, it was just a few days after the completion of this manuscript (in March 1963) that I first saw the book by Pontryagin and others, which deals with minimising problems through the same basic devices as presented herein." He names the two schools as working through "the same basic devices" without describing any contact or exchange between them before that point.

This is direct author testimony on the independent-discovery question that the vault had previously rested only on a historian's secondhand characterization — Breitner (2005) reports that Isaacs's 1951 RAND paper already contained "rudimentary precursors of the maximum principle, dynamic programming, and backward analysis" five years before Pontryagin's 1956 announcement (claim-isaacs-1951-rand-paper-precursors-maximum-principle). Isaacs's own 1963 near-miss does not establish the 1951-vs-1956 precedence question the vault left open at question-verify-pontryagin-1956-precedes-kelley-1960 (answered on other grounds, archived) — it is a separate data point, a decade later, about Isaacs's own manuscript relative to the published Pontryagin book rather than about the 1956 Doklady announcement's date or content.