Big old aluminum roasting pan / dutch oven, six quart size. The pan area measures about 12" x 9" x 4 1/2" deep.
It's been used and shows wear, and we can't make out the makers mark... something Cookware, I think?
But it's a good solid pot with a well-fitting lid - they don't make them like this nowadays. And it would make a great all-purpose pot for camping, you could easily roast a whole chicken!