A fascinating point, which I think too often goes without mention in photography. That is, if a photo is successful, how much of that success is due to qualities inherent to the subject ? And at what point (if any) should the subject be given attribution or credit for contributing significantly to a photo?
I think there is a spectrum of subject matter with nature and landscapes defining one end, and "art" at the other end. Falling in the middle are architecture and portraiture. For example, I think all subject matter in nature and landscape photography is considered to be public domain, and few photographers feel they need to give credit to the creator/Creator for using their/His intellectual property (tho perhaps we should ;-)
On the other hand, if I were to ever take a photograph of a photograph, or a painting, or any other fine art, I think it is only fair -- or at the very least, good manners -- to give credit to the artist (if known). Often these kinds of works are signed, and attribution is relatively easy.
To my way of thinking, models often make significant, active, creative contributions to the success of studio portraits, and I think they should be credited as co-creators of the work.
Architecture is less often signed, and I think of public buildings as being more towards the landscape-no-attribution-needed end of the spectrum. The same with a lot of craft like furniture.
Here, I think the subject of Atget's lion head door knocker falls into the craft/architecture category, and personally, I would probably not go to a lot of trouble researching its provenance. However, I do appreciate brief captions such as the title of the work and date, where known.