1018 is what I used for over 50 years and still do on the triggers,If you are going to mill it from 1/4 inch thick
material I would cut it to the length needed and then anneal it before milling.The hardening of the tumbler hole is not really needed but not a bad idea either so it's optional.The Davis mechanism is a better lock than the old ones seen in collections today.Good luck with this project.
Bob Roller