Tks BOAC, I believe the picture you mentioned was an F18, looked for it but without success. A stunning picture.
As you say, Crossunder, it's a question of refraction. But what was so fascinating was the extraordinarily fine definition of the shock front, and the compactness of it. Almost looked like a large diameter transparent washing line laid out along the wing. Was KT experimenting with a stealth HF aerial?.....