So the most likely problem with this GCV are the wood wedges were used to level the slates? If that is the case I thought GCVs had some sort of slate leveling screws or bolts?? I am not sure, I have never seen one yet other than in pictures.
The first GC4 frames didn't come out with the factory levelers, so yes, wedges had to be used to level the slates, nevertheless, had the slates been set up correctly, there never would have been a problem with the seams.