Well, having hunted around the Web, I've found photos of a 56(F) Sqn F-4 fitted with a yaw string, but not as shown in the previous photo. It seems to have been fitted at the base of the windscreen support, as Rp stated.
But I don't recall ever having seen one, let alone used it. How it was supposed to remain attached at supersonic speeds and cope with English rain, I cannot imagine.
I remember the 'danglometer', a little yaw string with a dayglo cone on the end on some glider though!