For mechanical upenders they usually have one 3/4 round shaped cradle which is often plalced on four motorized steel rollers. The ability of rotation comes from an electric powered gear motor which is driving a sprocket with single or dual chain.
There are two flat platforms for the basic cradle.
It’s not just for supporting the load when we talk about the platforms of the upender cradle. There are also handling device which designed for setting and removing the load. Such as V saddle which is in a good shape to fix coils and rolls. Sliding platform is for convenience to transfer the load.