Alternatives - Bismuth and a few others. I think "Hevi shot"
is another but you'll have to check.

Re proof marks, it will have been re proofed and shown as proofed for steel shot.

I find it unlikely that it will have been proofed for steel if an old english gun. The expense of sending it back to the UK or Europe is prohibitive unless it is well worth it.

Depending on the gun, better to take out the chokes
- others have done this.