The name "Seven" seems to have been given because of the auspiciousness of the seventh day of the week or lucky seven, but there is also a theory that "the number was increased to compete with the Seiko 5.
However, the actual reason is not clearly known.
Here's one the last great automatics from Citizen, circa 1978, caliber 8200 here housed in a Pakistani Air Force Aviator that shares the the same basic design as the 1969 Rally Custom