Given that I was on tracks 8-9 at the time, I was pretty sure it wasn’t a sign for 10-7, but it appears that A was the only lettered platform, and thus SNCF decided to do hexadecimal wrong.
I now on my next train to Nancy-Ville, which is a Z 26500-series double decker Alstom Coradia Duplex.
[Edited to add the photo of the sign]