These guys had a tank, and they didn’t want to go down there!
Think they were out of gas, old Twilight Zone episode. Their captain thinks they’re AWOL, but finds their names on the monument.
The tank and other modern era stuff got buried on site or carted off and officially forgotten if I had to go further with the plot.
There is a name on the monument of a person who was NOT killed there. Christopher Madsen was ordered to the 7th, when he was suddenly transferred to the 5th Cavalry. He witnessed Buffalo Bill kill Yellow Hand.
He later became a federal marshal to the Indian territories (Oklahoma) out of Fort Smith Arkansas.