I picked this cel up at the recent Comic-Con 2000 in San Diego, from the episode "The Koos Is Loose." It is made up of 4 cel layers (though I haven't taken enough of a look yet to see what each layer is). As the Certificate of Authenticity notes, the included background is a reproduction, rather than the original background.
I recently sold this cel (someone contacted me through email... he wanted to purchase it as a Christmas gift for his wife... it was a good cause, and I got a good price). But I've decided to leave the page... its a cool cel :-)