Actually I recently got a cel with a mark similar 
to that.  It's a two layer cel from Maho Tsuka 
Tai.  The bottom layer is A12 and the top is A12'.
The layers are stuck so I can't tell what they 
are of, but my guess is that the A12' top layer 
is a correction layer (If they use a wrong color 
or something the studio will sometimes just put a 
layer over top to cover it up).  So, maybe that 
is what it is for.  Does your cel have the 
apostrophe too?  What is the sketch from?