film – What is causing the light leak in my Hasselblad A12 back?

After finding very similar pictures of leaks online, and talking to an experienced repair guy, I found out that the leak is caused by an improperly closing auxiliary shutter.

The Hasselblad V series has two flaps on the back of the body, which need to make a proper seal together to prevent light that is coming through the viewfinder from reaching the film plane. In my body, these just don’t touch perfectly snugly.