Jay's solution requires that the axial hole in the blade boss has a blind end and an internal keyway. That would be rather difficult to manufacture, and therefore expensive. Weird that they would do it that way, which makes me suspect they didn't. The screw-slots in the end must also have a purpose: they cost money to make, so they were made for a reason.
Have you tried treating the end part as a screwed-in retainer for the blade boss? It may unscrew from a thread in the end of the crankshaft, leaving a crankshaft-sized hole in the blade boss, which would make the blade boss easy to manufacture. The retainer would then have a double purpose: it would have a male thread sticking out from the far side, to engage the internal crankshaft thread, and a female thread in the front, to receive the blade plate retaining bolt. If it works that way, all you need to do is unscrew the retainer, using the screwdriver slot in the end of it and holding the body of the blade boss to keep the crankshaft from rotating. You will need to make a special screwdriver tool to do it, if it works that way.
