Would love to see Model Y be the first candidate and I hope you are right.
But the only single form casting machine right now is the rear third section. It seems feasible that they would duplicate this first in Berlin and Austin. At that point, they could create a second mold machine. Since manufacturing the pack may take a lot of time, and the front section is a known and tested part, it seems reasonable for the second molding machine made be for the purposes of creating the single front section piece, before tackling the battery pack/middle section. Just my opinion.
FYI - I have almost no automotive industry experience. I'm coming at it from a standpoint as a business owner, project manager, and technician in a completely different industry.
PS. I played a small part if the team that created the first automated battery inspection machine and pack loader for Tesla's Model S & X.