It will be the chassis, Tony
 

The C8AX has the arched rear chassis as shown in the pics, which is much lighter than the straight rear chassis of the C15A.

I did read somewhere on MLU which model Chev ute chassis was used for the C8A and C8AX, and the fact both variants were built on the car assembly line at Oshawa. Note only the C8 had 13" wheels, C8A and C8AX had 16" wheels but with 9.25 - 16 tyres as standard.

The axles and components are much lighter too. I am working on a C15A at the moment and keep finding major differences to the C8AX I have owned for over 30 years!

The C15A rear hub bearing nuts and lock washers are all Ford pattern (and as used in Universal Carriers too!) with 8 sided nuts, and Ford style inner seals.

The C8AX rear hub bearing nuts are 6 sided, and the inner seal is felt.

Always learning...
Rob