Maybe someone else can have better luck. ^_^ I 
think I found the place where it *would* be, and 
I capped three frames... one before, one a smidge 
less than before, and one after. Might be wrong, 
though.

http://reccamuseum.topcities.com/bebop_screencap.h
tm

You can see the little part of the nunchuck (is 
that what it is? -_-) over his top shoulder in 
the last cap, and also in the cel. And perhaps, 
since his head stays still, you can't see his 
mouth change because there's nowhere for it to 
blur. But it's his bottom hand that I'm not sure 
about.

I also have a cel that I can't perfectly cap, 
either, because it blurs too quickly:

http://www.geocities.com/redkingshuri/for18.jpg
and the cap:
http://reccamuseum.topcities.com/for_22_15.jpg

So it's perfectly possible that the cel is legit. 
^_^ It doesn't seem like the kind of scene a 
fancel artist would try and pass off as 
genuine... especially if, as you say, he gave you 
a better scan.

Good luck, and hopefully, someone else can be 
more helpful! :o)
-Cres