Bill Gosper wrote:

As usual, friction supplies the time arrow. I have to believe this thing was discovered rather than invented. Reject surfboard? Lopsided kayak? Anyone know?

I'm sure it was discovered many, many times. I discovered it at the dinner table when I was in high school; it turns out the dinner knives in our everyday cutlery set were rattlebacks (probably not by design). They were great fun when dinnertime conversation lagged, though I think my siblings and parents found my habit of spinning knives increasingly annoying. Jim