Bessler's wheel was a prime mover because it extracted power from falling weights, but really I would like to say gravity!.
But within Bessler's wheel there must also be a prime mover which gets the wheel moving in the first instant. What part converts the source energy into mechanical energy? It must be the OOB weights. The wheel will rotate some amount as long as it is OOB, but the weight-lifter part of the mechanism won’t directly effect rotation, that effect only takes place after is has lifted the weights.