helper rear springs
 

Is there a way to find out from when helper springs where fitted to Chevrolet FAT`s ? I have found 12 cab`s with none on , but also 13 cab`s with none on .
There is a lot of weight on the rear springs when the Quad is loaded.

Parts manuals will usually list the different assemblies and a starting point for what serial number change in production. Your best bet would be to check the later parts books.

According to the CGT-03 parts list:
The rear auxiliary springs are listed as being introduced at serial 248--37091.
The second type body was introduced at approx. serial 248--01253.
So, there were 35838 vehicles produced between these changes but not all would have been CGT as I understand it with the serials being being shared between all production.
In other words- I believe there would have been some Cab 13 style Chevrolet Gun Tractors produced during the "1942" production period without auxiliary rear springs but I have no idea exactly how many.

What I noticed is that even the 12 cab quads have the holes pre drilled in the chassis for the helper spring brackets.