I do have an oversized cel that has registration 
holes both on the top and the right side.  From 
the above, I'm guessing this was the studio's 
effort to fix a similar problem.

Shame on the seller for not disclosing this 
before the sale, though!  I'd put this in the 
same category as cel creasing and cracks around 
registration holes ... damage done by the 
studio.  Even if your cel was hurt by 
professionals rather than amateurs, it's still a 
condition detail that you should have known 
before buying.