I feel certain that Andrey Vakhrulin is right to connect 1945 with "Anno Atomus" (taking his word on the Latin), or just as likely, "Atomic Age".  As a search of Google Books will show, the latter phrase immediately became a cliche.  An example in a date is "On the calendar, this is the year 1947. But it is also the year 2-AA, the second year of the atomic age."

Robert Ducharme Potter, Young People's Book of Atomic Energy (1946), p. 44.

For what my guess is worth, the ingenious connection to VN's American citizenship might have been a private, secondary meaning.

Jerry Friedman
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