Dave:
Make it out of a good grade of tool steel and you will be o.k......the 12 is a larger size than the 20-16 parts......a good machine shop would make you one for very little......and it would be machined correctly to size.....good hard tool steel is quite stout as is.....
I doubt it was ever a cast part, the cracked area probably crystalized and gives that appearance in the cracked surface....could have been an old file mark that continued growing into a crack....anybody's guess.....
I don't think the thumb lever yokes were ever case hardened.....they do not appear to be anyway.....and since the lever and stem are hardened, I doubt they would have designed two hardened pieces working against each other.....something has to flex a little.....
