Question: Denford Viceroy drill

As I said in another thread, I've just bought a Viceroy "20" pedestal drill. There's just one problem I need to resolve and hopefully someone
here will have an answer.
The spindle pulley fits over and drives a part (I'll call it the spindle pulley carrier) through which the splined spindle moves up and down. The carrier runs in the top bearing and is retained by a ring which screws into the casting. The problem is that the carrier has about 5mm of up/down float so the spindle pulley moves up/down by that amount; whilst it doesn't change the accuracy of the drill it feels odd as the quill is wound up and down so I'd like to fix it. Clearly I need to get the retaining ring out but someone has tried before by using a drift in the two holes, so they are bu&&ered.
How can I get this retaining ring out and what's underneath to control end float? (Presumably it's a right hand thread?)
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