Bought an old house. None of the duct is sealed, so I'm spending time taking care of it. Found lots of bad work including these. The first photo I was scratching my head why someone taped what should be only a short piece of straight and a 90, but there was other weird tape around, and nothing to block out air from where the dovetails are.
Second photo (hard to see my arrows) shows where I scraped off enough tape to show there are actually 4 pieces of duct to get air from the end cap to the bottom of the duct on the left side of the photo.