I thought I would pull this discussion out of the weekly photo discussion....
There are some interesting detail point that were mentioned in the thread, a great start.
Letterboards
Truck color (black is not really "rarer", just earlier).
Litho doors , versus swinging painted doors.
Here are some more identifying details:
Single or double roof cam-lock types...or o cam locks. (more on this in another post)
Roof ventilator Bumps vs. no bumps
Litho window trim colors (more on this in another post)
Set Truck vs. Flex Truck
These all help us identify the year and the set that Flyer used these cars.