So follow-up to the factory installed or accessory item question for heaters. The 1947 and 1948 books do not show the recirculating heater but the earlier version shown below. This is listed as an accessory item.
https://www.flickr.com/photos/advance-design_parts_co/15674350367/in/set-72157631868452754
The next photo shows the Fresh-Air (Air-Flow later) as option number 385. This would indicate that at least for AD's of 1947-48 this heater was a factory installed option.
https://www.flickr.com/photos/advance-design_parts_co/15257465364/in/set-72157631872034068
The 1949 book shows this heater had gone to the accessory list and for the remainder of the AD run this is how they appear.

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr