When you shoot steel out of a choke tube intended for lead you will get a tighter pattern, say, if you are using a modified with lead it will actually pattern as full with steel. So I think the reason it says steel only is that you will get a better pattern with steel. The lead may produce a bad pattern. It won't hurt that choke to shot lead out of it at all. Try the pattern with lead and then steel in that steel choke and look at the difference. I use steel designated choke tubes in my Browning o/u for lead, won't hurt a thing. Hope this helps. 
