That thought did pass thorough my mind. It's shiny and perhaps aluminium, clearly contrasting with the rusty steel of the interior of the fork wall, and I think it might have a thread in the central hole. But it just fits so perfectly all around, no gaps visible, it looks like it's literally welded, not corrosion welded.

Here's a zoom in.



I gave it a few whacks with a long rod from the top to see if it was movable, but not being sure I was a bit half hearted. But now I think it must be a wedge after all. So, I'm going in to town later in the week and will search for some penetrating oil and soak it and then either get it loose or, if it isn't a wedge, destroy the fork.

So stay tuned for the exciting denouement.