Can you confirm that cel and sketch are *exactly* 
the same, that nothing is missing, even the 
smallest black line ?

If not, I can just think that they decided to add 
another element on the picture between the time 
they made the sketch and the moment they made the 
cel, thus the cel became B3 instead of A3, and 
the new element was numbered A3. A "book layer" 
maybe ? It would explain why you don't have it: 
as there is only one, it usually gets sold with 
the B1 or END cel of the sequence.

It would make things easier if you could have a 
look at the video and tell us what the layer A3 
could represent.