yes - you won't get a 21mm knot into a 21mm hole. Don't the Muehle knots screw into the handle? That might be a safer, more straighter forward option. i usually buy the knot and then spec the handle bore size so I'm a bit wary of giving advice the other way around.
Anyone else chime in?
Ah dont worry about being too accurate, what's the worst that could happen?

Yeah, there is a plastic threaded insert inside the handle which the original knot screws into but I could quite easily remove it, if I go ahead with it even... I am going off the idea tbh, it owes me £25 so an extra £15 for a new knot and I could have got something half decent in the first place
I messaged Muhle earlier asking if it was normal for this model to have a 13mm glue bump and only hold enough lather for one pass?

I may just stick it on fleabay and get shot of the damn thing... Life's too short.