I have some doubts about a 1948 Model 12 stating that it's steel rated on the barrel. That was about the time that John Olin began experimenting with steel as an alternative to lead. I'm pretty sure that when they first started testing, they had very little idea of any possible dangers.
That said, I'm not sure I'd run steel through any gun with a full choke from that era if I were worried about damaging the gun. The only damage you'll likely suffer is a ring bulge behind the choke, but that's really only a cosmetic problem that won't effect the function of the gun. Were it me, I'd either have the gun opened to mod (about .020 to .025, no tighter) constriction or send it off to Mike Orlen and have it fitted for choke tubes.
I feel slightly sorry for a man who has never patterned his gun, who has no idea how far his chosen load will retain killing penetration. But I'm extremely sorry for the ducks he shoots at beyond the killing range of his gun and load - Bob Brister