Is the "weathervane probe" literally a small weathervane?
Yes.
Presumably with a repeater in the cockpit?
No. It's right in front of the pilot. It has beta-lights* on it so it can be seen in the dark.
And why don't they use an ultrasonic probe which would have no drag at higher speeds? For the uninitiated, ultrasonic weather vanes are more accurate and reliable than electromechanical vanes, and no more expensive - I have one on my boat after the old mechanical one blew off in a gale!
Not sure such things existed in the mid 60s, but even if they did they'd have been a poor choice because it would have entailed a significant installation. The windvane uses no power, has no wiring, weighs bog-all, has negligible drag and is dirt cheap.
*tritium plus some sort of fluorescent coating