Take a look at the production markings on the bottom of the Skeletor cel. If 'MUSTK' is written on it, then that means the cel is a stock cel (btw, She-Ra stock cels have 'PPSTK' instead, which is how I could tell the Shadow Weaver cels are stock cels).
If the cel has a number instead of 'STK' after 'MU', then that means the cel is from the numbered episode.
Stock cels are mostly generic pieces of animation that can be reused in other episodes wherever they may be suitable.(assuming stock cells means mulptiple cels for different animations , er, like main cel and then a little bit changed etc.)
MOTU and POP had a plethora of stock cels to keep production costs down.
I put my cels in polypropylene sleeves and keep the sleeved cels in folders. I do the same for backgrounds, though I don't sleeve the copy backgrounds.Getting a bit into the cels thing though, its just like owning a piece of the cartoon! I will have to find a good way top disply/store them.
How do keep them SGB?