In the UK I’d recommend Axesrus for guaranteed well-fitting knobs and theirs are available in both 18 and 24 spline versions. (There may be other suppliers who give similar full info but I haven’t come across any to date). They also fit well. Quite a few, even Gibson ones I’ve had, often only go partway down the shaft and sit quite high above the body. These one push all the way down if you want them to.