By my best knowledge (and those knowing more, 
please correct me!) the "apostrophe" you see by 
your cel's sequence number would be said "prime". 
As in "E1prime". It seems to be used to indicate 
images in a sequence that are secondary to other 
layers.

Since it's much easier to *show* this than try to 
explain it, here is a cel of mine with a prime 
(hoping all these pictures come out):



In the scene, the characters pull up and then 
hold still as a helicopter with searchlight moves 
overhead, changing only the shadowing, not the 
main image. The cel is *single* layer, marked 
only as B7. The sketch, of only the shadowing, is 
marked B7prime. Though I don't have the sketch 
for the main image, I suspect that it is marked 
to be used with multiple sequence numbers (for 
example, "B5-B8" or "B5-8") and was reused in 
combination with the prime layers to create the 
images with the different shadowing. Hence you 
would have a sequence that looked like this 
(using my hypothetical sequence numbers from 
above): B1, B2, B3, B4, B5-8 + B5', B5-8 + B6', 
B5-8 + B7', B5-8 + B8', B9end.

(The fact that this cel does NOT have the sketch 
of the main image actually supports my theory, 
since it is VERY rare for Psychic Force cels to 
come without sketches -- unless the sketch was 
reused and is thus with another cel it matches.)

I have seen another use for the prime notation, 
again in Psychic Force, where it seems to have 
been used to notate a change in the animation. In 
this sequence:



The layout of the layers is as follows: (left 
hand cel) Keith and the bed, the bandages, 
Keith's eyes, and (right hand cel) Keith plus his 
eyes and the bed, the bandages. Keith and the bed 
are the A layers, while his eyes are the B layer 
at the beginning of the sequence. However, the 
layer of bandages is between the A and B layers 
(where there shouldn't be a layer). The bandage 
layers are all marked as prime to the A layer. 
They were obviously added after the rest of the 
layers of the sequence were painted.

To support my theory, the A layers are animated 
with a scar on Keith's shoulder which is never 
seen in the anime (being under the bandages). The 
only reason to have painted it there is that it 
was *originally* meant to be seen. . . until the 
bandages were added. However, although a separate 
layer and supposed to lay directly over the A 
layer, the bandages couldn't be made the B layer 
since the eyes were already the B layer. (If made 
the C layer, it would be assumed that the bandage 
layers should go over the layer for the eyes, not 
between the Keith and eyes layers.) Thus they are 
prime (additions) to the A layer. For everyone's 
amusement, the scars:



(These cels have never been scanned without the 
bandage layers, so this is a bit of a treat. ^_^)

I hope this makes some bit of sense. I'm not 
completely clear on the use (all the uses) of 
prime layers myself, so I could be a little off, 
but this has been my experience with them. I wish 
I could remember off hand more of my cels with 
prime layers or sketches, because I *know* I have 
more. These were the only two that sprang 
immediately to mind, though.

That said, I'm not sure how your cel might be a 
prime, but am also unfamiliar with Card Captor 
Sakura and don't know how it was animated. In the 
end, it's an absolutely gorgeous cel of her in 
the Kitty costume! (My second favorite that I've 
seen. ^_^)

Many Sharp Smiles,
--Drac