Ah there it is.
The inside surface finish I thought would give away it's manufacturing method.
You were correct, these covers were either an afterthought or they knew only a small number were to be made.
That surface finish to me looks exactly like lost foam sand casting.
Which also explains the bumpy surface finish you said you were having to clean up on the outside as well.
Much cheaper to manufacture than how the normal covers are made. Al die casting.
In that closeup photo you can still see the styrofoam shape, looks exactly like packing material you would find around a TV or something in a box.
Is the main benefit of this cover the extra protruding cooling fins or is there something more? I recall some sort of cooling contraption that stuck out of the differential on euro spec models. Is this related to that?