Chokes marked "For Lead Only" are normally not hardened or made from hardened steel. The manufacturer is really the only one who knows what materials are used (bar stock, seamless tubing, etc.), how it's made and many other variables that can cause him to restrict it to lead shot. Because he knows more than I can know about his choke, I follow the "rules" on his choke.
If you have any concern about the safety of your gun, you should take it to a qualified gunsmith and let him take a look at it.
"If he's out of range, it just means that he has another day and so do you."