You won't find a gun made prior to 2005 that carries the proofmarks "steel shot" or the fleur de lys, unless it has been subsequently reproofed. Those proofmarks weren't added until then. However, if a gun carries the superior/magnum proof, then as far as pressure goes, there should not be a problem. But, as noted above, that leaves the issue of choke, and the possible issue of barrel walls thin enough to end up with a ring bulge. I'd say that if the gun is superior proof and if the choke is no tighter than mod, and if you shoot steel no larger than 4's at moderate velocity, you should be OK. But if it were me, I'd probably go with something like Kent's Bismuth.
Steel shot proof requires no higher pressure than superior proof. The only difference is that the proof loads contain steel shot.
