by single piece i mean that the tapered end is integral to the hub instead of being separate like with the zenoah or conversions where the flywheel is turned down.
yes, end of second paragraph the hub is removed from the lathe or mill as concentricity is not an issue at that stage. the 19mm counterbore described here is the access for the nut that holds the hub to the crank. on commercial engines this will usually be a special nut that also includes the 10mm prop centering shaft on the opposite end. i just use the regular nut and then a plug with the centering shaft is inserted into the bore and held with set screws.
third paragraph describes the steel mandrel used to hold the hub for the rest of the machining. this is just a tool and not part of the hub.
the plug with center shaft can be seen in the attachment. the three flutes cut in the plug provide seats for the set screws without marring the outer surface. it is a very close fit in the hub, any dings and it would not be coming out again.