My understanding is that some Omega Race games came with pot-based spinners, and some came with regular optical spinners. They both have the same knob, but they look different underneath. Both the pot and optical version spin freely, the full 360 degrees.
Check these pics, both are from the undersides of Omega Race control panels for sale at
www.quarterarcade.com:


So far as I know, the pot-based ones are of little use. You can inteface them the same as any other pot-based device, but MAME doesn't know what to do with a 360 degree spinning pot. Unless I'm wrong about that.