AHA!! I just located the missing finger!

It looked like she was barring the 5th fret with her 2nd finger, but that would really be doing things the hard way. No, what she's doing in the photo is supporting her index (1st) finger by adding her second finger on top of it. From the observer's viewpoint, her 2nd finger is eclipsing her 1st.

It suddenly hit me because one of my guitar students is just starting to learn bar chords, and she keeps trying to support her 1st finger with her 2nd. A lot of beginners do that, but quickly learn that in a lot of bar chords, they're going to need that 2nd finger somewhere else. It's just a matter of time and practice, building up strength in the barring finger.

All of which indicates that Joan was a beginner once. That should be encouraging!

Don Firth