The strongest point I have for thinking your cel 
isn't a fancel is because of the airbrushing on 
it.  I have *NEVER* seen a fancel that used 
airbrushing, nevermind good airbrushing.  And 
I've seen and done a hell of a lot of fancels ^_^

And the possibility of erasing a cel and the 
sketch, and redoing both?  aaa!  
Nevernevernever.  It would be easier to buy pre-
punched cel acetate and just do a fake sketch... 
or no sketch and all and just say it didn't come 
with a sketch since many cels don't anyway.  

For the most part, us fancel artists are a lazy 
lot when it comes to faking details, which is why 
people on this forum catch the not-too-scrupulous 
ones pretty often.

And lastly.... It's still a really nice cel.  
Does it matter to you that much where it came 
from?