I don't think hornets eat pollen and they don't make honey.They eat insects and spiders. I wonder if it was cool out and perhaps the sun was warming the steel slightly and they were on the steel or warmth?
It was 45 years or so ago so damn if I remember the paint but it was sunny. I thought they might have been attracted to the warm metal. IDK. But I'm talking many thousands of wasps. Not flying, not eating, as far as I noticed. Just chilling, like a huge organism or entity. Geez...I watch too many creature feature movies
But I'm sure the Rangers that man the towers have seen it before, and may know the answer.