But I think that the prime means either that the 
cel is a correction layer, or a corrected cel.  
That is, it's the second rendition of the 
picture, and the one that was actually used for 
the animation.

I've seen more than one sequence, where the 
entire scene was animated with corrected cels or 
with correction layers on each cel, because the 
director didn't like the "look" of the painted 
scene, and wanted to apply some uniform change of 
look to each cel in the scene.  The "Dragon 
Riding" sequence on the opening credits of Gokudo 
is one of these sequences.