Hi Dave,
We build these horns in my shop hear in Ridgefield WA, this is how it?s supposed to be, the weld is left off in this spot for clearance to the bolt that connects the horns together to the bearing. Sometimes if the tube is not perfectly square on the end there can be a small gap between the stamping and the end of the tube, also on occasion the gap gets missed and welded closed so it is possible you could get one with the gap and one without. In any case your golden, keep pounding those rivets!
Russ