First heater is a GM truck heater common from 1941 thru phase out in 1947 both 1st and 2nd series. Second is a car heater prior to dash mounted control units in 1949 I think. These are also sometimes seen in Pontiac, Olds and Buick's. Third is an aftermarket, common brands were Firestone, B.F. Goodrich, Arvin, Montgomery-Ward and others. In addition to the first heater correct AD truck heaters are versions of these.
http://www.flickr.com/photos/advance-design_parts_co/8129448404/in/set-72157631868452754Variations include color, doors, aluminum plate vs. decal, vents, and switches.
