Is a screw missing from the left carb top?
Certainly looks that way. It also appears that the carb tops are rotated counterclockwise from the proper orientation by 90°. This might alter the transient fuel/air mix a bit, and I've always considered it an indicator that the person who assembled the carbs didn't quite understand what they were doing. On the other hand, the Huffaker race cars rotated the tops of their highly modified carbs by 180°, so maybe it's all a Super Secret Racer Trick TM that we're not supposed to notice.