I've used steel shot for many years, and I've gone to a 20ga for most of my hunting including waterfowling. I find the 20ga steel to work well, at least as well as you can expect steel to work. I still prefer #3 for most applications (ducks over decoys). #BB for Mallards and geese otherwise.
I'd be really reluctant to shoot steel shot through any s/s or o/u, regardless of age or linage.
Even a dedicated waterfowling gun with replaceable choke tubes is going to eventually bulge the choke in front of the choke constriction. (I've bulged several) If this is a soldered barrel such as your high-grade o/u, you may ruin an expensive gun.
As such, I only shoot Bismuth, and tungsten-matrix through my s/s, even though it's of recent mfg, and has choke tubes. (CZ-USA Golden Pheasant).
I bought a 'cheapy' Charles Daly pump in 20ga and painted it to match the foilage in the areas I hunt as my 20ga waterfowler. It uses Rem-choke tubes and I bought it used for $125.00 at a pawn shop, so if it is lost overboard or I bend a barrel taking a spill wading a beaver swamp, no great loss. I've aquired several choke tubes on clearances, ect. so, choke tubes are expendable...... A $$$$ shotgun, isn't....
I also have a Remington 870 Express in 12ga to back up my M1187SP camo, for the same reason.
It's a sad day that it's cheaper to by another shotgun and shoot steel than to use either Bismuth or Tungsten alloys through a "GOOD" gun that you don't want to mess up....... But,thats where we're at now, and it's no use looking back....
Heck, in my neck of the woods, it's cheaper to pay the fines for using lead shot, than to buy/shoot the "legal" ammo. And thats coming from someone who used to be on the "other end" of the law ......
But then, I prefer to avoid the "hassle", so I buy the ammo after the end of the season when it's marked down. I haven't needed to buy "expensive" ammo for several years now....Still shooting ammo I bought over 10+ years ago...