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.http://books.google.com/books?id=2NfqAAAAMAAJ&q=%22second+year+of+the+atomic+age%22&dq=%22second+year+of+the+atomic+age%22&hl=en&ei=DtgETo6aL8LegQfzlsG_DQ&sa=X&oi=book_result&ct=result&resnum=1&sqi=2&ved=0CC0Q6AEwAA
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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