That appears to be the lens barrel or a spacer barrel. It should be screwed into the shutter. They can become stuck or corroded in place.
1. Grip the barrel with a strap wrench and try to loosen it. Be careful not to bend or deform the barrel. You may need to remove the shutter from the lens board and grip it with a second strap wrench.
2. Heat your oven to its lowest setting usually 150F/65C for gas or 175F/79C for electric. When the oven reaches temperature turn it off and place the shutter in the center of the oven on a rack and let it sit there for 15 to 20 minutes. Remove the shutter from the oven and try to unscrew the barrel from the shutter. Should be standard threads.
3. Seal the shutter in a plastic bag and freeze it. Once frozen thaw and try to unscrew the barrel.
4. Last resort apply penetrating oil around the outside of the barrel at the shutter and allow to soak in for 12 to 24 hours.