Actually, I got it started from the inside over a tightly packed shot bag, then 90% of it was done from the outside shrinking the corners over a scrap of 2x4 with the corners rough rasped round. After the picture I did tap out the dimples from the inside over a chunk of lead though.
What got me thinking about it was seeing a couple of sheet plates attached with nails. I was thinking it would be possible to shape the butt, inlet and nail down the comb extension, then form the rest down over the butt. It seemed reasonable, but you're right Rich, there would be no way to anneal it like that. I was concentrating on the forming process and didn't think about that.