My Mum bought this at the brocante yesterday. Apparently it's a gratte pain - a bread scraper. If you leave the bread in the oven too long and the outside is a bit burned you just scrape it off with this tool. It's a gorgeous thing - I love the texture of the metal.
This is an interesting site with lots of fascinating old French tools and utensils. It's in French but you can have it in somewhat approximate English too - good enough to get the gist!