The French Antique Guessing Game started out with a BANG!! The first comment nailed it, guessing correctly with:
Kit said, "It was for supporting panes of glass while the glazier was working on the window frame. These days glaziers drive around with huge great frames on the back of their vans to keep the glass safe, but in those days glass panes were small and windows made up of several panes, so this would help him measure out each pane and keep them safe at the same time."
I must also say that Anne Lenz answer a châssis de fenêtre, was close, but this item is not the window structure but the holder of the glass for the structure. I looked up châssis de fenêtre and found an interesting site sharing historical fact... I love this kind of stuff. I even think my Brother Mat (who turns his head to brocante) would like this as it is about French historical architecture.
Zosia's answer had le laughing out loud as I could see this as a confessional window, because I spent many Saturday mornings in front of one as a kid. "Dear Father I have sinned..." Zosia's comment was, "It's a part of a confessional. This is the "window" through which you confess your sins to the priest. You can rest your hands on the shelf. The shown "trellis" would have a denser caning or wiring in between to obscure the priest and the sinner."
Glazier - Vitrier. Size #2, Elite (the 2-3/4 inch - 7 cm size). From Marcel Carbonel, Premiere Santon Makers, Marseilles, France.
When I read what Donna wrote, "I suspect that if I found the right Santon, I would find this on it's back." I had to see if I could find one, and sure enough the French nativity set has a Santon Glazier!
Once the plate is ready to please the custom then up to you.
The most creative answer goes to Julie M. when she wrote:
"Comment dit-on "tic tac toe" en Français?"
Thank you for guessing, your responses make this worth doing, and allows me to try to find old French things that might stump all of us one day.