We have gone off topic. This is a good topic, as it presents a mystery to many people. As I previously said, the steel at the turn of the century was sufficiently strong for any firearm made. Manufactures use what is available and it's 4140 now. It is about 50% stronger but that delta is never brought to bear, except in certain actions. The Merkel is an example. The 20 gauge action is heat treated 4130 to resist stretching. Why use a shotgun action in the fist place? Cost.

So owning an shooting a pre-war double is just as safe as it is today with a new double.