I'm not positive, so look for confirmation about this, but I believe Glock is switching over to two-stage guide rods for all their models. This is the kind that has two thicknesses with a different spring on each instead of one spring down a guide rod that is the same thickness for its entire length.
Does it look like this?
They are supposed to make recoil smoother, especially in shorter-barreled guns.
"An armed society is a polite society." Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
William S. Burroughs