I was very lucky the other day to see a very rare bookplate. It is an ex-libris for Leonard Smithers: bookman, publisher to the 1890s decadents including Oscar Wilde, peddler in all manner of erotica and "amatory unorthodoxy". It is something of a mystery. Only one other is known to exist. There is no monogram nor signature, which is unsurprising given Smithers's reputation, it would be unfortunate if you were an artist to have your name permanently inside the kind of books that Smithers might have on his shelves. It bears a resemblance to the work of too many artists of the period to use the style as any clue to authorship. The fact that both copies which exist are printed on the same size paper but it is an impracticable size for sticking in a book, and also the fact that is not known to have been used by Smithers in any of the books from his own library, all suggests to me that this was a proof or an example that was never printed in any numbers. Perhaps Smithers didn't like it or simply commissioned it as an artistic exercise rather then to be used in actual books.
[my apologies for the photo quality, I was reduced to using the camera on my phone when I encountered this bookplate and had no scanner nor proper camera with me]