That is indeed a lid to a Dutch Oven.Can you read any large numbers on it?Can you see any design,like a Phoenix bird?I also find those,and they are always broken,I have figured they couldnt handle the constant direct heat,or during the night they absorbed moisture hen when put in the fire the exploded!!I do not think it is 1700's,but I do commonly find them in 1850's sites.Here is the Phoenix Stove works emblem,which is quite rare,and I thought I had several pics of complete ones I have dug but I dont know where they are,I can go take photos of them if you wish