Identical to the SHS version. XT is a new SHS OEM. On the outside of the bucking are 3 small molded ridges designed to better hold the rubber in the chamber. Inside the bucking has a small molded mound. Rubber quality seems slighly improved on the non improved seal model.
The durometer refers to the rubber hardness. The higher the number the harder the rubber is. For high FPS 450+ 70 is often better. For lower FPS 350 FPS 60 is generally better.