I finally figured out where the Yu-Gi-Oh cels at 
Nick's came from--they're from an obscure 
1998 Toei Animation TV series that is a really 
close adaptation of Takahashi's original 
manga.  I think it only ran for only one season.  
This series is called Yu-Gi-Oh, whereas the 
new one (which is CG) is called Yu-Gi-Oh 
Digital Monsters in Japan.  The two series are 
totally different (different voice actors, Kaiba 
has green hair, etc.).  About the sequence 
numbers--they appear to only be on the top 
most layer (I have one cel with no sequence 
number on the B layer, (A layer is missing) but 
a number on the C layer.