Material suggestions?

[Highpower]

I pulled the ratchet off to get dimensions. It is keyed to the pinion as I thought I remembered. I was really curious about what I have here, so I stroked it with a file a few times. No problem shaving off bright shiny chips so it's definitely not hardened.  So I'll just go with the annealed 4140.
I'm going to be hosed anyway because I think the largest broach I have is 3/8", and this key way appears to be 10mm.   

Yeah, yeah, get the file out - I know...