You have to look what You have, there's way more variables than the ones above.
From the more common in instruments variety, ~6mm there's:
6mm solid shaft
6mm splined (and usually, but not necessarily, split as well) shaft.
6mm beveled shaft.
1/4" solid shaft
1/4" splined (and usually, but not necessarily, split as well) shaft.
1/4" beveled shaft.
The beveled ones are less common than the others, found mainly on concentric potentiometers IME, but that's still more than enough variables to get it wrong.
IMLE the make and model without the exact date or year of manufacture doesn't ensure correct fit.
So, IME better to measure than guess.