Thanks for the replies
My shotgun (actually my father's) is a 50ish year old Zoli. It says nitro proved on the barrel, which apparently means it's okay for steel shot but not the high performance stuff.
It does have a stamp of what looks like a fleur de lis, but different from that Sideshow posted, it is inside a upside-down heart shape.
I'm pretty sure the bottom barrel is full choke, so I think I'll go with bismuth, but at $100/box I would probably be better off to buy a new shotgun!
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