It was an option in those days. Additionally, the COE was pretty rare on its own. People didn't order options as easily, so most of the COE's that had van bodies or something like that couldn't use the corner windows to any advantage, so they didn't order them,
My dad had a 48 COE wrecker with a Holmes Wrecker Body. That's what I learned to drive a truck in. The corner windows were cool, but they really didn't serve much purpose. The driver's side was too close to use and the passenger side was at enough of an angle that it didn't really help.
I guess that that's my take on why there weren't more built.
