This is a baseplate and axle for the Triple gear
(full size version). The axle has a 5mm diameter hole in the bottom, for fitting onto a motor. Here is what it looks like powering the triple gear:
This version of the model is for the polished plastic materials. The polishing media can get stuck in the 5mm diameter hole of the other version: this version has a tiny bar across the opening of the hole to prevent this. The bar can be cut away before the helix is fit onto a motor.