The easy way to tell the difference between early and late rear struts is to look at the mounting bracket at the bottom. Early on left, late on right.
View attachment 16767
The mount on top is harder to see because you pretty much have to disassemble it. The early rear struts used the same hardware that the front struts use. Because the rear struts don't turn when the steering wheel is turned some of these parts are unnecessary. This is a layout of the parts that mount the top of the strut to the body on the early struts (front and rear).
View attachment 16769
These parts are not used on the late rear struts.
View attachment 16770
The reason the fiber washer and seal are not used is because they are needed to allow the front strut to turn. The rears don't turn, so the late rear struts were modified to eliminate them. The late struts don't need a separate cone because it is built into the upper spring mount.
The early top spring mount is on the left, the late top spring mount with the built in cone is on the right.
View attachment 16771
The separate cone mounts on the shaft as below on the early struts.
View attachment 16772
The late strut top spring mount doesn't use the separate cone so the top of the shafts are different. The early strut rod is on top.
View attachment 16768
Bottom line - either the early or late strut will mount just fine at the top, as long as all the proper parts are used.
In my experience either the early or late strut will also work on the bottom because I have always had enough clearance to the CV joint. YMMV
(Maybe Fiat made the change not to allow more clearance, but to get the rear higher for bumper laws? Who knows?)