From-
Slammers, Stacks & Hitters,
A Collector's Guide To Milk Cap Bottle Caps

By Adam Woog
Penguin Books USA, Inc., 375 Hudson Street, New York, New York 10014, 1995

"Back in the 1930s and 40s, money was scarce in Hawaii, and kids played with anything cheap - including the little cardboard caps that sealed milk bottles.   Milk wasn't available in cartons then like it is today.  In the '50s and '60s , it became cool to collect these caps. In 1991, a schoolteacher name Blossom Galbiso started using old milk bottle caps in her classroom.  Kids began playing with them and, as Mrs. Galbiso says, "something magical happened."
Suddenly, cap were everywhere - at banks, burger joint, even at dentists' offices.   Kids all over Hawaii went milk cap crazy.  Now milk cap mania is sweeping the world! ....."

"This has to be the world's most valuable cap.   Designed by Dr. Rodney Chang, it sports 14-karat gold, three diamonds and a sterling silver staple.  Yours for a mere $1,200."

Note - Rodney Chang started Slap Caps Company, dedicated to creating fine arts on such caps, or "pogs" as they were called locally.   Original drawings and paintings were done as miniatures on such circular objects.  

Chang created the FIRST two sided design cap, which created a local sensation, generating sales and interest in his "Bug Collection Series."

As a kid Chang avidly played "milk kavas" with friends at school and at home. So the 50's as mentioned in the book, was not merely a time of collector interest but active play by that generation of early Baby Boomers.