These are used in conjunction with steel chains to secure heavy loads to anything that will haul them. Your find might have been used in some like manner to maybe tighten a sail , tarp or secure some other object. I know that is a stab in the dark,but maybe my comments will open up an avenue to understand what it really is. I just visited France. That part of the world is beautiful and full of treasure. Good luck with your work as an archaeologist! Our history is so very fascinating and important. I would appreciate if you kept me in the loop if you find what it really is. Once again, GOOD LUCK!!!!!