From This Might Be A Wiki
The Flex-A-Tone is a 1920s percussion instrument consisting of wooden balls on a spring attached to a flexible metal sheet . When the instrument is shaken, the balls strike the sheet producing a sound similar to a musical saw. 
Marty Beller plays the Flex-A-Tone on the Spine version of Stalk Of Wheat. It's played by Dan Miller during live shows.