They could be fastened on top or underneath, just depended on the design. The oldest one I've ever seen was from a Ford dealership from 1931. How I know that is the grandson of the original owner of that Model A still has the car, with the original tags and the topper still with the car, along with the original bill of sale and all of the documentation of everything ever done to the car. It's a really neat car with amazing history.
My old '46 Chevy (now fumblin46's truck) had the remnants of the original dealer sticker (similar to the above) on the side of the cowl, and it was an original OK truck.
