I'd also say its quite likely this is an opening 
cel.... there is another Evangelion cel that has 
been on the market that is also from the opening 
(one with a full shot of the Eva and shinji 
superimposed on it), so it wouldn't be 
surprising that this was floating around.  Also, 
I used to own another Unit 01 mecha cel that was 
from an actual episode....and I know this is 
just a scan, but the quality level of that cel 
definitely seems more like an opening cel, as 
the quality of the cels from the actual show 
don't seem quite as high.

-Justin