Your sear/tumbler would be a perfect use of Kasenite or one of the more recent versions of surface hardening compounds.
Quick and easily done in the shop. I've rehardened many, many sears, triggers, and other parts with the stuff over the years and it's always worked well.
No need to go very deep w/a case on small thin parts like a trigger/sear tip or a tumbler notch. Too deep and you can weaken the thin part as it gets hardened all the way through or nearly so and is glass brittle.
I usually just case harden the sear tip by dipping that portion of the part in the compound and letting it 'take' to the part.
No need to harden the whole thing IMO and chance getting the really thin areas thru hardened and brittle.
I did that a few times.
The tumbler I'd probably harden the whole part instead of trying to segregate and harden just the notch. But again there's no need to cook the thing for 20min to get a decent case on the part for what is expected of it.
Any thin cutouts on the tumbler like for a stirrup and pin can get over hardened just like the thin trigger/sear edge with the use of the stuff.
Just my experience.
