Okay, first of all let me say I am NOT an expert 
and got this from no authorized sources!  But that 
doesn't look like an E to me... if you look at it 
it doesn't seem to have the top line of the E, 
although the circle intersects it at that point... 
it may possibly be a pretty messily written kanji 
for "ue" that means upper, top, above, rising, 
etc.  I also know thst sometimes marking like that 
are used for corrections of errors on the initial 
cel... are the layers stuck?  If they're not you 
may check and see if that upper layer contains a 
"retouched" version of anything on the other 
layers... if the matchign sketch doesn't say end 
and the number isn't even circled, I would say it 
is neither an end cel nor any key cel... but 
again, that's just a guess!  ^_^

Isn't it interesting to look at cels and try to 
figure out exactly how that part of the show was 
put together?  Nice cel BTW!  ^_^

Tifa