The extended choke tube is a Polychoke II. If you don't know about polychokes, they have been around for probably 40 years or so. Normally they are permanently attached, but a while back they came out with the II's, which is the polychoke with a section that is as long as and threaded like a traditional tube. It just screws into the barrel, like I had on the Topper Deluxe Barrel.
The polychoke is a choke that allows you to twist a collar (without tools) and open or close the choke from about 2 constrictions more open than cylinder choke through x-full. It has 9 settings in between. I love them and have been using them for years. I always have the right choke with me, and it takes about 5 seconds to adjust it.
I bought mine from Cabelas, but you can also get it from Poly-Choke.com. They also install the permanent ones if you want.