A legitimate production cel can be missing it's 
sequence number for a variety of reasons.  If 
it's a cut oversized cel, it might be that the 
part with the sequence number was cut off 
(sometimes normal sized cels have been cut as 
such as well).  Also, depending on the type of 
pen used, it might have come off over time due to 
normal wear and tear (or maybe if a previous 
owner removed it on purpose).  Sometimes, the 
registration holes and production markings are 
also on another strip of acetate, taped onto the 
cel when there's not enough room otherwise.  It 
may have been a piece like that removed as well.

Hope this helps!

Keys