From the pictures, Dan, my guess would be that the remaining section may have been pressed into the shutter housing as a means of reducing manufacturing cost. If the section of the internally-threaded black ring at the front overlaps the front surface of the shutter housing, that would be an indicator of the direction of the press-in operation. If that retainer is actually pressed in, you might be able to remove it by tapping from the rear, using a flat-tipped pin punch and gentle taps with a small hammer. I'd try to support the front of the shutter housing by drilling a hole slightly larger than the outside diameter of the retaining ring.
It appears these cameras were not designed to be serviced or repaired, so there's a risk of damage to the parts you want to keep for your pinhole.
Before taking my "bigger hammer" approach, you might want to contact a camera repair pro to see if they have any suggestions. Carol Flutot is a member here, and might have a better idea.