[MENTION=3339]andrewzorz[/MENTION] The Toonie reference was to the diameter of the cylinder.
In image 1 the lockout spring is pushing up on the adapter and which is not screwed in. The spring is meant to push down on the lockout when the knob is in place. In picture two the adapter is screwed down in place (or near in place). Once you put the knob or anything else you use the adapter as a lock nut.
That all said this particular adapter has a 1/8" (~2mm) lip see picture:
You could grind that down to have a flat surface.
Here is a picture pre knob install.
Now if you want I could take mine off tonight and snap a picture and take a measure should you wish. (might at lunch if I am bored)