The above rules can be combined as in this case:

If the letter is followed by 1 and END (i.e. 
A1END, B-1 end, C1e, D-1E, etc. . .), it's key.  
The only thing that's extra special about these 
cels is that it's the only cel in its sequence 
(for that particular layer).

Below is an example of an A1 END cel.  It's 
somewhat unusual in that the cel number is not in 
a corner, but you get the idea.