Place cursor over the image.  Please note the near top edge is parallel to the bottom edge of the crop.  You can see an approximately 1-degree discrepancy with the far top edge.  This is entirely natural for rectangular instruments whose strings are arranged at a slight diagonal.  Among other things, the diagonal arrangement allows the lever arms of the bass-most keys to be as long as possible, thus optimizing the touch.  The arrangement also allows the key touch surfaces to be placed entirely within the simple rectangle and, while routing the strings to avoid the keywell, enables the maker to optimize the bass string lengths as well as the arrangement of the bass strings on the soundboard.   In this instance, the resulting deformation is gratifyingly small for 30+ years under tension.