Gravely began building rear-engine riders in the 60's with the first "traditional" looking unit in 1967, I think. They built a surreal looking rider called the "Westchester" before that--1964, perhaps. All of you Gravely afficiandos are welcome to jump in with more clear facts than I have.
The first Gravelys were walk-behind two-wheel tractors built starting back in the early 1900s, and they are still built today. The first models replaced plow horses and looked like a horse-drawn plow with an engine set low between the handle bars. Pretty unique engineering story and design. Ariens bought them in 1982 and discontinued the Pro-G riders this year.
They have been owned by several companies, including Studebaker, who owned them when the 424 rider was introduced in 1967. The rear engine design really puts full power to the ground.
Can't wait to get it up to snuff, find some decals, and have a good professional paint job put on her.