bolded the important part.
You making this post have documented a history the next of kin could use for a claim, if they wanted to. It has not been long enough to go public domain by your statement.
If really concerned about it, ask around in the family.
I would add a page though. "created by XYZ in 19xx" should at least be on there. possibly a reprinted by XYZ as well.
that way at least the content creator is acknowledged.