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While all the addresses were changed in a few months, the conversion took several years (and is still taking place). According to Davis, much of the old material had alpha-numeric addresses, which had to run concurrently with the new six-digit addresses, so mail was running on dual systems with both addresses, as it was being converted. Half the mail had new addresses, half had the old addresses. While most of the alpha-numeric addresses have been converted, remnants of the old addresses remain, requiring the dual systems to still be run today.