The Manufacturer would not print the file because it has too many faces and it needs to be a smooth surface everything, just like the car.
(File size is suspiciously big)
1)-while you do that, please build a hollow cylinder into the center of the knob:
very simple: outer diameter 10mm, inner diameter 1/4inch; (this will represent the brass piece - the cylinder is 9mm high
-from the the bottom of the brass piece to the bottom of the knob it should be 3mm
-from the top of the cylinder to top of knob it should be 2mm (in between is all plastic)
all that totals up to 14mm height of knob:
2) the outer ring of the bottom of the knob should only be 1mm wide, then there should be a little step of about 1/4mm and then the slop goes down until it hits the "brass cylinder"
3)pls send dwg and stl file (and maya file for backup)
I can send pictures to clarify...