Craft Supplies lists the bushings as the same part
number as for the regular slimline twist pen.
The instructions I got with the ONE I ordered
show the small bushing in the center,and some lame
reference to "considerable design freedom".[:0]
It is easier to me to get the regular slim kit,
and a set of 7mm european tubes,use the longer tube
on the lower.As far as sizes in the center,I shoot
for about .470-.490 on the upper and .410-.430 on lower.
Hope this helps.