This photo is truly a lucky shot. Since I had neglected to bring a shutter release cable for my camera, I was using the camera's timer to trigger the shot, allowing me to take my hands off the camera and minimize vibration during the shot. During the timer count-down, the camera swings the viewing mirror out of the way, so that it is not possible to see the subject in the camera viewfinder.
After the shutter was triggered for this shot, I looked into the viewfinder and found two lines crossing the disk of the sun and assumed that some distant power lines had gotten into the shot, ruining the photo. I continued shooting until the transit ended, thinking no more about the "ruined" shot. But days later after the film was processed, I discovered the bonus! I doubt that I would have been able to snap the shot this well even if I had seen the plane approaching the sun's image!