Kevin,
I think it is bolted on underneath. There are build book photos on here like this one here showing that. But I have never attempted it. I think the bolts go through or into the wooden inner arch with maybe a nut on the inside & are then covered on the inside by leather card.
Gambalunga's build book show more.

The parts book you can download from BHM shows this & MGB0027 seems to be a bolt.
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Hopefully someone with hands on experience will come along.