At a contest in Region V, i.e., southeastern United States. It was either at Cordele or Perry:

Glider pilot: "Towplane, 5 knots faster, please."
Tow Pilot "Which towplane?"
Glider pilot: "The Pawnee."
Tow Pilot: "Which Pawnee?"
Glider pilot: "The white one."
Tow Pilot: "Which white one?"

There were at least four white Pawnees towing at the contest, all of which had visible identifying letters/numbers. They finally figured out which towplane should fly a little faster.

Ray
W8