Sorting out little problems with my mill continues, with help from this forum!

As the DRO installation has now finished I am looking at the drive pulley on the quill spindle. It has about 5mm of vertical movement resulting in the belt not aligning properly. I have an exploded drawing of the assembly but it does not show in enough detail how its all held together. There is one grub screw on the pulley, with this removed I would have expected the pulley to just slide off the spindle. It just seems to abruptly stop on what you would think is a shoulder on the spindle, except the spindle has no shoulder!

So I wonder if any members here have the same mill? Should this pulley 'float', and has anyone taken one of these apart?

The second photo is of my Dad, he's 91, always loved tools, machinery etc, has lost the sight in one eye and the other is not really that good, but was determined to get on the steps to see / feel what was going on with the pulley. He sometimes has a lot more patience,and determination than me!