Never heard of them. Jerry, you are SO much younger than I am.

Wikipedia:

"Clackers, also known as Clankers, Ker-Bangers, and numerous other names, were toys popular in the late 1960s and early 1970s.

"Clackers were taken off the market in the United States and Canada when reports came out of children becoming injured while playing with them. Fairly heavy and fast-moving, and made of hard acrylic plastic, the balls would occasionally shatter upon striking each other."