Many thanks to David Elliott who dissected a hopeless Smith barrel showing the hook and rib extension are not one piece, and are indeed brazed.

"The extension and hook are two separate pieces, not one as the X-ray seems to show. The barrels are milled to the required angles and the extension and hook are fit to create the assembly. The extension has a locating pin that fits into the hole created when the barrels are put together. The hook comes in from the bottom and the assembly is complete."