Now they're made of dense plastic. My kid calls them the Ow-Ow toy, because I'd hit my arm while showing him how to make it clack at the bottom and top. Ow, ow, ow.
It became very popular in Egypt few years ago. Kids called it Sisi's balls, which angered the authorities that tried to crack down on its sale.
https://www.al-monitor.com/originals/2017/11/sisis-balls-gets-confiscated-by-egyptian-police.html