From Key's
Little Anime Cel FAQ
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What's the deal with Ah! My Goddess!?
	
Sadly the Ah!/Oh! My Goddess! OVA series had a lot
of unmarked studio-made reproductions. These tend
to vary in only the minutest details from what
appears on the screen. Be careful when you're
buying that perfect Belldandy! There's a higher
chance that normal than she might be a reproduction.
---

This was probably one of those. From what I
understand they were done by the studio at the
exact same time as the 'real' cels so, they have
the same paint colors and the lines have the same
fading. The only way to truely tell is to look at
the cel in question while either looking at the
animation book with that scene in it or comparing
the TV image.