My suggestion of a possible source for the famous Kelly Johnson case (discussed below) was a very densely packed lenticular cloud. In searching for photographs of clouds that matched the one I was proposing, I had to look far and wide. Most lenticulars are nowhere near dense enough for the scenario I proposed. I finally found the excellent photograph by Mark Meyer that I used in the piece. I love that image but have always hoped that I would find even better ones.
Recently some photos that seem to be of a lenticular cloud taken in Brazil have surfaced that feature the extremely smooth edges that I suggested may have fooled Kelly Johnson.
Here is one of them:
Such a beautiful photo. And so very like the description given by Johnson and Crew.
Here is a discussion that features more of the photos of the same cloud.
Thanks to Leonardo Araujo for permission to use the photo here.