I have a similar situation with a Mahou Tsukai 
Tai! cel as well. I didn't notice the "A22'" mark 
on mine until Faris pointed it out. My layers 
aren't stuck and they do reveal that the layer 
with the apostrophe is the correction layer. 

So, either that's something specific to this show 
or it's a general way of marking correction 
layers.

I learn something new everyday about my cels! 
Thanks for asking the question.

Wendy