While not necessarily true for the early vickies (I do not know the details), a frame was put in to production to handle the body mount all slant window bodies.
The 2/3 body mount bracket was moved back such that the front hole is located where the rear hole was located. They put 3 holes in the frame just ahead of the rear running board bracket so that a #6 mount point could be mounted on the frame. My guess is that the bracket was put on while the frame was being set up for the body so the rivets will be like the running board rivet, natural.
See Steve Plucker's website for frame changes and dates.