This was a typo, im meant to put Atra. The source may not be too reliable, i got it from Russel Adams book. I remember that it may contain few errors, one such error is on same subject matter on the Atra page 267 bottom page: he spells misspells Australian test razor- Autralian Test Razor.Is "Attra" a typo or a historical artifact? Nowadays the spelling is "Atra".
This is a confusing story: apparently Gillette used the name "Atra" as an internal code name for the Trac II, the first generation of twin-blade cartridge. Then a few years later they used Atra again, as the public name for the modern pivoting Atra / Contour product.
McKibben starts his version of the story in 1964: http://books.google.com/books?id=YCldvmXq25EC&pg=PA67. I think McKibben himself got confused about the details, sometimes distinguishing the Rex from the Atra and sometimes conflating them. But it looks like the original Atra (developed 1964-71) was basically a code name for the Trac II, which launched in October 1971 according to McKibben. The modern pivot-head Atra / Contour was the next generation and came out ca. 1976.