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The distinction is how the two were original attached. The wider, deeper channel on the left was originally riveted in place and the material that held fast the tacks was inserted into that channel. The channel on the right was added after the outer top material was tacked in place to the channel on the left and trimmed. It was tacked in place over the installed top material and a formed rubber bead was inserted to cover up the heads of the tacks used to attach it.