Guns made after the late 1980's were specifically designed for use with steel shot. MOST older guns choked modified or more open will shoot steel without damage even though not specifically designed for it. Some will not, especially most of the older doubles. The problem is that the steel was very thin in most of the older doubles to reduce weight. I'm not sure about this gun though.
I wouldn't use steel in an older gun with any sentimental value just to be safe. I use an old pre WW-2 double from time to time. There are other non toxic options that are perfectly safe. They are expensive, but less expensive than ruining a famly heirloom.