Ted, are you certain that such "bits" as sears and springs, screws, triggers, bridles are or ever were castings?

I used to be very "into" Spanish guns and rebuilt several for my own use and never encountered any evidence whatsoever for soft steel. Having never seen a soft-steeled Spanish gun (nor has any of the far more serious collectors I know) is why I suspect that if there indeed ever were any such guns they have all been used up and thrown away. Guns made in the past 20 years or so certainly have never been accused of suffering from soft steel.

I suspect that "soft Spanish steel" is just one of those gun legends that won't die gracefully.