Those Repack stencil rectangles are also discussed in the SPH&TS book on painting & lettering SP and PFE freight cars (page 127). The drawing for the stencil was issued in March 1938. I have never noticed a stencil with data in both rectangles.
I don't have a photo of both rectangles in use, but I do have a photo of the lower rectangle in use (below). I don't know the photographer; my friend Clark Bauer sent me this photo.
Note also that a clerk has tacked some cards in the same area!
