Hi moon_sushi,

I'd say it's a 99.99% chance that it is a 
legitimate production cel.  Cels from the 
Sailomoon series are notorious for being trimmed, 
unfortunately.  And when they are cut, many times 
it is trimmed to where the paint starts (and many 
times they also cut off part of the paint X\).  
So it is not uncommon for the sequence number to 
be trimmed off as well. 

Hope this helps.
~Luna-Art >^.^<