How to track legacy numbering?

I have quite a few comics (The Amazing Spider-man, Vol. 2) that where logged a few years ago and the Issue numberfield was populated with a combination of the Variant and the Legacy issue number. I updated one of the issues from Core and now the Variant is just the variant. Where do I now find the legacy number for this issue? I’m having difficulty finding this info and I don’t want to update the other issues until I understand how to include Legacy issue data.