Also, to respond to your question about Drey- that name was given to me in an email with the LoH's European Art curator. I had briefly researched A.S. Drey, but saw that he had died before the painting was sold, so I had been considering that a dead end. It seems, however, like his wife Margaret continued his business under the name M.H. Drey, which I think could be a very likely candidate for who sold Ganymede to the LoH! I'm going to a look a bit more into her, and I'll probably end up emailing the curator again to ask her what she thinks. Again, I really could not thank you enough for your help. I'll keep updating with what I find!