3. Door Windows
1947.2 -1950
The first years of the Series had single-piece door glass.

1951 - 1955.1
For the 1951 model year through the end of the series in 1955, the AD trucks featured "Ventipanes" -- door vent windows.
This feature replaced the side cowl vent (see next section).

4. Side Cowl Vent
1947.2 -1950
The '47-'50 AD trucks sport a side cowl vent in addition to the top cowl vent. The top cowl vent is found on all Advance Design trucks.

1951 - 1955.1
For 1951, the side cowl vent was eliminated and its function replaced by the ventipanes (discussed above)

Source
Stovebolt Tech Tip: Advance Design Spotting Guide

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC