Not sure about vent panes just being used on light and medium duty models.
I got it from a pdf on gmheritagecenter.com
You can read it here >
THE FIRST CENTURY OF GMC TRUCK HISTORY Scroll down to: 1951, read 3rd paragraph.
Scroll to 1954 read 4th paragraph, says The Twin Hydra-Matic combination proved to be unreliable.
Most of them were later replaced by manual transmissions without cost to the owners.
The pdf looks to be mostly about GMC trucks but includes Chevrolet trucks in some years.
Should be fairly accurate coming from the GM Heritage Center.
Interesting Stuff.
