Mine has an o-ring on the cranked roller which turns the idle roller. I'm 99% sure it said in the instructions supplied that the O-Ring doesn't need to be replaced if it breaks as the grain will turn the other roller when crushing.
I guess they just put it on in the factory check both rollers turn.
The o-ring is there for assembly, I cut mine off before using the mill so it wouldn't end up in my crush. The grains make the second roller spin. It is amazing how fast you can burn through 28# of grains with a drill on full speed, and awesome crush and efficiency.