It's a 16 years old vehicle. If you don't own it from new lots might have happend during this period.
FIAT / SEVEL and the converters messed a lot with the wheels, models etc. Rear ALKO axis will won't fit FIAT original specs. This wasn't standard, depends on the vehicle type and converters' options.
Try checking with the VIN number on the forum
ePER (link on the bottom of the forum banner, search window top left) in the van options (should be listed when you push the blue 'All' button). It shall tell you the chassis type it left the SEVEL factory with
In this
ePER section:
HOME
FIAT COMMERCIAL
NUOVO DUCATO
2J NUOVO DUCATO 2006 (2006-2014)
443 SUSPENSION AND WHEELS
44316 WHEELS
44316/00 WHEELS
you shall also find the rims it left the factory on.
Then, before you purchase anything, compare it with what you have on the vehicle, as that might have been changed during those 16 years.