I don't know about all of them (being removable) but on the early AD's (47-48) and similar year GM cars it was actually a design setting. My 47-48 AD manuals show it as an operational feature.
https://www.flickr.com/photos/advance-design_parts_co/11597806626/in/set-72157631868452754
https://www.flickr.com/photos/advance-design_parts_co/11597349184/in/set-72157631868452754