Well, I watched episode 200 again, and I checked 
the scene at the end frame by frame.  I notice 
from your Rubberslug gallery that there are 
actually two layers to the cel.  I found what 
looked like a perfect match for the Mamoru 
layer, but none for Usagi.  That makes me think 
that either the two layers didn't originally go 
together, or they were together but came from a 
different point in Stars (I haven't seen it all).

I'll try to answer your other question, but 
given my limited experience, I could be wrong.  
I believe the circled sequence number means it 
is a key cel - one drawn by an experienced 
animator as opposed to the in-betweeners that 
draw the frames "in between" key cels.  
The "end" means it is the last cel in the 
sequence, which I think is also usually done by 
a key animator.  Just so you know, the apparent 
match for Mamoru at the end of Ep. 200 is the 
first frame of the sequence but definitely not 
the last.  So if it is Mamoru's layer that is A1 
End, that eliminates that scene from contention.

I hope I answered your question correctly.  
Sorry I wasn't able to give you a more 
definitive ID.

Finally, I don't have any sequence mates for 
your cel, but I do have one from Ep. 200 of 
Serenity, so here it is: