Hi Glenn,
I was lucky enough to purchase a 1915 runabout /roadster a year and a half ago from a local collector. Part of the inventory that came with the car was the last known owners list for the Simplex line of cars. I have a copy of it in PDF and could email it to you if you like. It was last compiled in 1991...
In researching the car and the owners list, I was able to track down the son of the fellow that compiled the directory. His father, now deceased, was apparently very taken with the Simplex brand (sounded like almost an obsession) and he spent YEARS chasing down and maintaining a list of these cars and who was in possession of them.
Over the course of the last year, I've put a new radiator in my car... the core having been made in England and then a shop in P.A. having pulled out the old rad and installed the new core in the shell. While VERY expensive, I am very happy with the work that was done and the 'ol gal is just purring down the highway.. !!!
My car is the earliest known 1915 in existence, Ser #2045.
roger