Some kind of file or float

I don't know what this is. My friend found it in his basement. Apparently it had belonged to his dad.
It has curved grooves like a float. There is a tension rod that causes the file or float to bend slightly.
There are no identifying numbers or names.
I'm wondering, because of the curve if it could done a ferries tool.
The float is 14 inches long, the entire tool is 16 inches long.