I built a camera not long ago and had this same kind of problem I put on a lens hood and stopped it but, I wasn't having it even in the shade. I even have this indoors. Always in exactly the same place. My guess is that it isnt a flare but a leak. Now if it is a leak is it eminating from the shutter blades since it is in the middle or the mount since it runs out the bottom. It seems to spread from the center out but Im not sure. What do you think... all the other pictures from this test roll have the same thing to a greater this is the lessor amount .