I've never seen a Japanese cel that *does* come 
with any kind of Certificate of Authenticity. 
Only US cels have those on any kind of regular 
basis, and even then not always. I wouldn't count 
the lack of one as an indicator that a cel isn't 
authentic.

There are some indicators of authenticity, like 
the Studio Periot seal you mention and the 
impression stamp that AIC puts on their cels, but 
not *all* authentic cels from those studios have 
those things - only some do. So really the only 
thing those are good for is helping to ID unknown 
cels, since that indicates at least what studio 
made the cel.

Many Sharp Smiles,
--Drac