Ed Huntress wrote:

snip

The channel was broached.


Probably after being hot-punched in a secondary forging operation. The whole
body could be forged and punched in two hits.

Thanks for the thoughts. That was my own best guess.

I'm a little surprised that it's possible to broach a deep channel with
such thin walls (only about 3/32" thick) without everything getting
mangled. Guess the key must be getting it hot enough.

Best wishes,

Chris