This is a stainless steel chimney cap and top damper. Top dampers are a way of effectively handling cold air down drafts that occur on masonry chimneys in the winter. the plate fits snugly against the fiberglass gasket.
We also use top dampers to prevent bees from making hives in chimneys, which is a common occurrence.
This chimney repair is a repair we did as a result of the La Habra earthquake.