I'll add..
I agree with Rick partially. I believe the change goes in when they changed fans. The original cars had a flater fan. The later a deeper fan. When Rick and me put the new style fan/fan clutch on my car- the fan blades are not fully in the shroud. That's not going to give you the best airflow, but if your fan clutch was trashed, it will still seem like a huge improvement with the old shroud.
To fully cover the fan, the shroud must be deeper and hence the 2 -piece fan shroud was born. As noted above, it as the bonus of being removable without taking off the fan & fan clutch assy.

Rick: Do tell is the p/n's above good for the 300E/400E/500E?

Michael