I just looked closer at the photo you posted and that's exactly the same as the mounting bolt I have. Your derailleur will function perfectly on a standard 10mm threaded hanger with no modifications to the bolt or the hanger. I funny part is that I've been using these derailleurs for 30+ years and I've never even seen a Simplex dropout (with the derailleur removed that is) in person.

In my opinion, SLJ's are not only beautiful, they're also some of the nicest shifting of all vintage derailleurs - certainly better than anything Campy offered in the 70's. It's hard to tell from your photo of the back, but I think yours is a 5500 "T", which is the medium-cage model. It will handle up to a 30t cog with ease. A Jack Taylor with metal Simplex derailleurs sounds super yummy! You'll have to post pictures when you're done. Get busy...