The simple fact of the matter is these guns were only designed for lead shot, period.
The steel used in shot today is as hard or harder than steel used in older barrels. Steel does not swage down as readily as lead in a choke possibly resulting in stresses for which the gun was not designed.
I have a 1939 M37 16 gauge Full choke. I use Kent TM or Bismuth while jump shooting ducks with this gun.-Dick