I have not used any without set screws, so hesitated to try to help, hoping someone with that experience might. The knobs I have are very close to 1/4", I think sold as 1/4" but some are probably metric equivalents. All with set screws. And I have used the stock potentiometers and several stepped attenuators.
Having no set screw, do you think there is a sort of spring mechanism running the length of the connection to the shaft that puts pressure on the connection? Don't know. Hopefully no one glued them... just have no experience with ones without screws, but I guess I would be inclined to carefully, with a little side to side pressure, pull or pry one upward and see if it starts to give in a way that "feels" right, not too forceful.
Hopefully someone with more conclusive info will respond.
That said, I suppose they may be stock knobs, and if so, I would call Sarah and she can probably help you.
Good luck. I don't like those gold ones on the MK!V much myself.
EDIT: I just went to the workshop to check the Blue Torii and ZStage, both with gold knobs, and they all have set screws too...