Belt tensioner assembly? Keep us updated.
-Vivek
These will support a belt, but not tension it.
Wetzel
That looks like a hole pattern for a globe motor, or possibly a CIM. So a gearbox of some kind? Or simply a motor mount?
Here they are assembled as a suspension system for the robot. When loaded, the tension on the spring makes it so the plate and the block are pretty close to squared up.
Wetzel