This is both a general question and a specific question, or should I say questions?

There are a lot of downloadable plans out there. I'm talking about old designs, not new ones. How would I figure out if a given design is copyrighted?

More specifically, there are some plans out there for Jodel D-11. These are obviously images (possibly scans, or maybe photos) of very old plans in the original French. I believe (though I'm not sure) that some are downloadable for free, and they are also available as part of collections of old aircraft plans that some people sell for a nominal charge at places like ebay (maybe $10 or so). I think there are also places that sell individual set of plans for under $100.

The question arises whether these Jodel D-11 plans are under copyright. I'm not talking about re-drawn plans and the like, but rather images of original plans. Were the plans copyrighted originally? Has the copyright expired? I know there were changes in the copyright laws some years ago, and the whole topic of copyrights is complicated, what with the various dates, changes in laws, and domestic (US) vs. foreign content creators, registered vs. unregistered copyrights, works with copyright claims on the work vs. those without, works created 1978 vs. those created afterward, old (pre-1978) copyrights that may have been renewed or not renewed, and so forth, and it's all very confusing.

Obviously, if you want to discuss philosophical points regarding creator rights feel free to do so, but I'm more interested in the specific question of what is legal and what is not.

I realize that there are re-drawn plans converted to English, and there are also plans at places like Sandalwood and Avions Jodel. I'm not so interested in discussing those because I assume they are modern incarnations of the plans rather than originals, though I could be wrong.

I did find a document from the US Copyright Office that discusses duration of copyrights. It is nominally written in plain English, but even that is confusing. However, as I read the document it seems to me that the original D-11 plans are probably not currently under copyright unless the original copyright (if it ever actually existed) was renewed. Here a link to the document. https://www.copyright.gov/circs/circ15a.pdf.

Returning to the original general question, this issue probably applies to other old plans as well, not just Jodel.