I made this simple sweater almost five years ago now. I created a basic raglan sleeve pattern with extended front panels to create the overlap. Garter stitch finishes the neck, sleeves and hem. No one can really argue that the two vintage shell buttons are not the key feature of this sweater.
I adapted the basic raglan top down method by adding stitches in the front to create an overlap. If you want to learn more about top down raglans check out this page. There's lots more info out there if you do a web search.