One can only conclude that it was for use with an automatic welder of some sort so that the position of the lug is constant to the path of the welding rod, and used on a production basis. On those that I have done I have just held the body in a pair of aluminium jaws in the vise and TIG welded the lug. They were only ever done on a one-at-a-time basis, and hand fitted. This jig could also be used to position the bodies for a milling operation to provide a constant dimension in the refurb process.