I don't think Mission made a 747 arm. Grace made a 747, Mission made the 774 and 775 arms.
If it is 774, the below link will give you some info. BTW, it's an excellent arm, if it's still in good shape.
http://www.vinylengine.com/library/mission/774-(original).shtml
If it is 774, the below link will give you some info. BTW, it's an excellent arm, if it's still in good shape.
http://www.vinylengine.com/library/mission/774-(original).shtml