I should really figure out a good way to describe the tube details…. There have been a few variations over time and it is difficult to describe them.
Note that the center tube is different on this brick. The inner wall of the central tube is less tapered. I expect this is done to ensure that the clutch power of studs connected to the center tube is similar to the clutch power of studs connected to the corners of the brick.
[Original post and comments on Flickr.]