I've heard the same thing, that not all frames
from XG have their sequence numbers.  A friend in
Japan with a lovely group cel (w/number) said that
she'd seen one other from the same sequence as
here, and it had no number.  So maybe only the key
frames are numbered, or some such?

One way or another, congrats to both of you for
finding XG cels!  :)  :)