This tool is useful for wood turning when you need to quickly find the center of a raw log or already roughed out piece of wood :
So it's a quarter of a disk, with side walls, at square angle.
At 45°, there is a blade, inlaid in the sole. It's actually a long jigsaw blade I have reground.
To use it, you put the end of the log, beat with the mallet, turn roughly by 90° and beat again
And here is what you obtain, the center is (approximately) marked.