[drc]

A few comments.

It is a printing plate. There are metal proofs, which are rare themselves, and this isn't a metal proof.

Cards back then were either photoengravings or lithographs. Both printing plates would be metal. I can't 100% tell from the pics what this are.

It is true that with lithography they had/have a transfer process that would flip the image (via in between sheet of paper) during printing. Thus lithographic printing plates can have normal or backward image and text.

The wooden backing was common on old commercial printing plates.

I doubt it is from a newspaper, and it would seem unlikely from a magazine. Newspaper plates are almost always flat, and magazine plates I think are also usually flat. If it's a lithography printing plate, it is not for a periodical, including book. Periodicals back then weren't lithographs.

I can't say it's for trading cards, but it is a neat item.