Browning introduced the Purbaugh tubes in 1965 as "Supertubes." They're about 14" long and made of aluminum. I didn't know they continued as late as the mid-70's. I have a set for all gauges. Each pair is color coded by gauge and marked Upper and Lower. Supposedly, they were fitted to a specific gun and weren't supposed to be used in another. They pattern a bit tighter than regular Superposed guns of the same bore, based on my "exhaustive" test. They ARE a bit tricky to install, what with having to change ejectors and all that. But if installed correctly, they function fine and I've never had any problems with the ejectors.
