This tiny dowel is key to mounting the motor. because it converts the square-section that I've always preferred for constructing my drones to a circular. If you're interested in the history of steam locomotives ~ many of which were pioneered hereabouts ~ you will know that among their most important elements is the 'throat plate', a contoured steel section that adapted the fire-box (which was square, as its name suggests) to the boiler (which was round to contain the pressure of super-heated steam).
