This original Harley-Davidson tool (plate) came in a two different sizes. One for smaller bore pistons(above) and a larger one for big-twin pistons. It's designed to check rod straightness, and possibly used for assembling cylinders (my application). It's slotted where it fits around the connecting rod and serves as a solid base to hold the piston while you assemble the cylinder, with rings compressed. It gives you an "extra hand" and eliminates problems such as nicked pistons(on cylinder studs), rings popping out, damaged rings, etc. If you see one at a swap meet, buy it !