Thanks a lot. One possibility is the change might have been performed at the same time
the diffuser below the trunk was changed to provide access to the tow hitch for the export version.
Anyway the single piece is very bulky, takes a lot of space for storage, and is not easy to manipulate by one person.
I wonder during the production line if the single piece of carpet cover with sub trunk, was entered from the back,
from the roof before putting the top window, or from one of the rear doors?
And to install it faster, there might have been two workers because this one piece didn't slide easily between the two wheelhouses.
So I suspect the rear trunk opening was used, with one person holding it from outside, and another person pulling it from inside.
I cannot imagine a robot able to put it inside, unless the robot has multiple arms.
Using two separate parts simplify everything, even when a body shop need to replace the rear panel, there is no need to remove the back seat.
Note:
@MMcKay if you have access to the Model Y's EPC, can you check if the Model Y casting in two pieces is now made in one single piece?