Looks like there is some kind of special hub that allowed the pulley to turn without turning the wheel. Otherwise he would have to jack up the left rear wheel only, which is more likely.
The belt is off the pulley on that. The operator may have used it on the drive to the location then disconnected it. A rather novel way to run an air compressor if that's what it is. The tank sets right behind the rear wheel.
A lot of Model T's were set up this way to drive equipment, such as saws to cut firewood. The one rear wheel was jacked up. A flat belt was run to the equipment.