I haven't found much info on this site regarding diffusers. I don't know the difference between a good one and a bad one. Here's a photo typical of the ones at my grandma's house, probably original, ca 1948. This one is the longest, 8 ft. They all have the same profile, 5in at the back, 3 in at the base. The duct opening is in the center an is 2"x12". I'm guessing these are no good, right?