Golfcartman, I don't have a Simplicity Wonder-Boy, but I have done a little research on them as a part of my Simplicity history quest. I am doing the series on my blog, and I haven't finished working up to the Wonder-Boy time period. I do have the history in progress, though, on my website www.oldpathsequipment.com
From my notes so far, it seems that the Wonder-Boy was the first rear engine mower of all time.
The name came from William J. Niederkorn, Simplicity's executive and a marketing genius. He dubbed it the Wonder-Boy after a nickname his mother give him as a child.
The model evolved into the Simplicity Coronet series.
Maybe someone more familiar with Simplicity will correct me, but I did not know that they ever used a Clinton on a rider. I know for certain that Briggs was used on all early models for the Landlord/Broadmoor/Sovereign, and I always thought that Simplicity stuck with Briggs from the first, switching to Kohler only later. But I will stand corrected, perhaps (since you have a 1959, the first year of manufacture) you have an exception.
Are you familiar with simpletractors.com? They have a forum just for old Simplicity machines, and they will be able to assist you much better than I ever could. My only condition is that if you find answers, post them here so I can stand corrected!
I have attached an ad for the Wonder-Boy. It's a bit newer than your model, since they replaced the hardpan seat and tiller with a soft seat and a steering wheel. It has to be after 1964 since the Broadmoor was only introduced then.
Ben W. in VA