Solid tube, extra position notch, and different. - Strike Industries Advanced Receiver Extension Buffer Tube

Yup it's a buffer tube. Screw it on, shove in your spring and weight and move on. This one is a bit more expensive, but the looks, fit, and finish is quite nice. Up to you if it's worth it. Generally these things are interchangeable and one is as good as the other. I went with this because I was doing other blue line parts from strike. The missus found a better fit on this one than standard buffer tubes because of the extra notch, and a black one went to replace the one on hers.