If the sump has been replaced, quite likely on a car of that age, it may have been fitted with an aftermarket sump which may have a hex head screw instead of the hex key plug on the Fiat sump.
If you are working on the floor be extra careful about supporting the car. There is a big sideways pull needed to shift the plug/screw so don't have the car in the air with only a jack. It will fall off.