Dan Kubin discovered that the harder rubber was causing some spitting issues. He decided to use a slightly softer rubber to help combat the spitting issues some folks have. They are designed to hold up to the aggressive nature of hard hitting machines such as Sidewinders.
Made from the most durable rubber Dan could find. They are just hard enough.
He modified the outside profile for a little bit of a radius at the shoulder. This will help keep it strong where it needs it most. (Similar time how the old black grommets were).
They have a tapered inside diameter, so as it is pressed onto the post the nipple spreads itself around the needle loop for a secure fit. Dan's favorite part of this style of secure fit is that the grommet isn't stuck to the needle when removing it from the machine.