This is the bottom of the seat tube extender that fits into the seat tube - and a carbon 27.2mm seat post fits into the top. Because I'm routing the rear brake cable inside the seat tube, I wanted some way to stop the seat post being accidentally lowered enough to foul - and possibly damage - the brake cable. Brazing a small nut to the inside of the insert tube will limit the downward travel of the seat post, so that it can't foul the brake cable. The insert tube's potential downward travel is limited by an external button that I brazed on to fit a notch in the top of the seat tube. Now there's no way the brake cable can be damaged by the seat post or the insert being pushed down far enough to hit it!