Ok, I'll try opening it up just for fun really. It is one half of the zoom ring that is movable, and it does shift the whole lens in some strange way.
Like you said Mr Rusty, it is cheap, but a great size, and I was going to use it both on my OM's and my 5D as a standard to-go-lens when I know the light will be enough.

So I will try finding some loose screws this weekend then!