@bertsmobile1And I will make that number 3
In any case it still loos like it is going to drive out from the top
So put the nut back on the top And give it a few healthy "taps" and see if it shifts.
The only thing I have seen anything like that is the Ariens Zoom spindle housing which has a similar looking collar top & bottom which threads onto the spindle but that one looks like the spindle pushes through,
The holes where he removed the Allen head screws may be for a spanner wrench. Ariens/Gravely does have spindles that look similar. But, we can’t see the entire assembly and we do not know the model number and serial number of the machine. Oregon and Rotary assemblies that cross to the 10161HD can be purchased for about $75 each.