The Air-Flow heater was an R.P.O. In 1947 after that year it dropped to the accessories list for dealer or owner installation.
https://www.flickr.com/photos/advance-design_parts_co/17056970488/in/album-72157651718017649/
1947-early 48 leftovers, more in GMC than Chevys it seems? Two versions, deluxe had the defroster attachment as shown. This heater appears in the Salesman Data Book in 1947.
https://www.flickr.com/photos/advance-design_parts_co/9201965046/in/album-72157631868452754/
Two most common installs 47-52 (53-55 1st Series have internal door)
https://www.flickr.com/photos/advance-design_parts_co/8129448404/in/album-72157631868452754/
https://www.flickr.com/photos/advance-design_parts_co/8129470099/in/album-72157631868452754/