Looks to be 2.5mm, 27.2. Bear in mind the stiffness of a tube is proportional to the square of its diameter, if you need a fatter post. I have about 7-8” exposed, hanging out, not sure how to avoid the obvious entendre.
The fact it says 20mm on it suggests different setbacks are/were available... Looks like it could go as low as 15mm. Those teeth are nice and fine, too.