Folk Dancing Kids In Costumes, 1920?United States PC, Images turn silver with re-positioning of card- What type of film?

A "JUST FOR FUN" KIND OF A REAL PHOTO POST-CARD. THE IMAGES TURN GHOSTLY SILVER IF YOU TURN THE CARD UP FROM THE BOTTOM.
(OOOPS CAPS WERE ON)--.
What type of process it this?
What county do the kids or teens represent??
Card will enlarge.
The seated people in the background look to be dressed for 1915-20.