Back in late July of last year (the 28th specifically), I got ahold of a cel of Lampy from one of the two Brave Little Toaster sequels. I've went through both films frame by frame and didn't find a single instance of the cel. I may've missed it, but I believe it may be a cel that didn't make it into the final movie.
It's believed to have been intended to appear at the timestamp 0:59:43 in The Brave Little Toaster Goes To Mars but I have no proof to back up that theory other than the similar appearance art-wise.
For the past months I've been searching for this frame with a couple other people but have come up with nothing. All the information I have is that it is most likely from The Brave Little Toaster Goes To Mars since the artists did not draw Lampy's eyes in the way they appear in the cel until GTM was released. The cel was mislabeled and sold to me as a cel from the 1987 original, this was confirmed to be untrue since the certificate reads that it came from Animation Legends, who only carry cels from TTR and GTM.
The cel is authentic, pictured below is the photo provided by the Ebay seller which also contains the Certificate of Authenticity, as well as a photo I took myself after it had been framed;