This came with a bike I bought a few months ago, I’ve only just got round to sorting it out.
Anyway, never seen this thing before, what is it?
The tiles it is sat on are approx 150mm across, it is marked 175mm up tp 270mm, with an o ring at the measurments.

[insert brand name you dislike] butt plug? 😯

Thankyou for your useful insight, it is clear you know far more than me about such things.

What bike did it come with? And does it have any annoying creaks or rattles?

I don’t know what it is but it could be something very similar to the Chris King adjustable headset pre-loader (bottom of this page):

https://chrisking.com/headsets/hds_onepointfive.

Looks like some kind of a head lock.

World’s worst dropper seatpost?

Yes, Mildred has it, I think it was inside the fork tube when bought.

So what is the advantage of using it?
Thanks.

What forks are you using, looks like it could be for altering travel on the fork.

The bottom half of a steerer tube lenghterator/cutterizor of some ilk, left in there by accident