Thanks Doug. I have managed to remove gear knob which I believe is unique to Roadster 3.7 which also has a spring loaded lifting sleeve as reverse detent. Both you and Jon describe a ring below knob to be screwed down but unless I misunderstand haven't such. See pic showing underside of mine.
![[Linked Image]](https://i.imgur.com/nrcboYn.jpg)
This shows a plastic insert to engage with threads which has been a little mauled by unscrewing. Three grub screws 120° apart (two can just be seen if you zoom in) through inner metal part are obviously to secure. I can only assume the outer part of knob unscrews from inner to allow access to grub screws but where to grip

Note also how face of inner part has been roughly machined by MMC to allow knob to screw further onto shaft I presume.
Final pic of gear stick and detent although probably not relevant.
![[Linked Image]](https://i.imgur.com/MjQ92yn.jpg)