Different front fenders? Boom support legs?
I presume there may be other changes which I do not know about (not a truck expert …)
Also a future release.
“The Series 1 used 11.25 - 20 tires and lacked the rear drag winch. The Series 2 had its spare tires mounting changed to one diagonally behind the cab and the other on the passengers side of the crane base. The Series 3 which lacked the front and rear trailer connecters found on earlier models were produced for Lend-Lease to the Commonwealth nations. The Series 4 had a Gar Wood US5G crane readily identifiable by its curved boom. To ad to the confusion the M1A1 was also known as the M1 Series 5.”
https://www.armyvehicles.dk/wardlafrance1000.htm